On Fri, 13 Sep 2019 at 17:22, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Fri, Sep 13, 2019 at 6:00 AM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> >
> > On Fri, 30 Aug 2019 at 03:06, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > >
> > > UEFI 2.8 defines an EFI_MEMORY_SP attribute bit to augment the
> > > interpretation of the EFI Memory Types as "reserved for a specific
> > > purpose".
> > >
> > > The proposed Linux behavior for specific purpose memory is that it is
> > > reserved for direct-access (device-dax) by default and not available for
> > > any kernel usage, not even as an OOM fallback. Later, through udev
> > > scripts or another init mechanism, these device-dax claimed ranges can
> > > be reconfigured and hot-added to the available System-RAM with a unique
> > > node identifier. This device-dax management scheme implements "soft" in
> > > the "soft reserved" designation by allowing some or all of the
> > > reservation to be recovered as typical memory. This policy can be
> > > disabled at compile-time with CONFIG_EFI_SOFT_RESERVE=n, or runtime with
> > > efi=nosoftreserve.
> > >
> > > This patch introduces 2 new concepts at once given the entanglement
> > > between early boot enumeration relative to memory that can optionally be
> > > reserved from the kernel page allocator by default. The new concepts
> > > are:
> > >
> > > - E820_TYPE_SOFT_RESERVED: Upon detecting the EFI_MEMORY_SP
> > > attribute on EFI_CONVENTIONAL memory, update the E820 map with this
> > > new type. Only perform this classification if the
> > > CONFIG_EFI_SOFT_RESERVE=y policy is enabled, otherwise treat it as
> > > typical ram.
> > >
> > > - IORES_DESC_SOFT_RESERVED: Add a new I/O resource descriptor for
> > > a device driver to search iomem resources for application specific
> > > memory. Teach the iomem code to identify such ranges as "Soft Reserved".
> > >
> > > A follow-on change integrates parsing of the ACPI HMAT to identify the
> > > node and sub-range boundaries of EFI_MEMORY_SP designated memory. For
> > > now, just identify and reserve memory of this type.
> > >
> > > The translation of EFI_CONVENTIONAL_MEMORY + EFI_MEMORY_SP to "soft
> > > reserved" is x86/E820-only, but other archs could choose to publish
> > > IORES_DESC_SOFT_RESERVED resources from their platform-firmware memory
> > > map handlers. Other EFI-capable platforms would need to go audit their
> > > local usages of EFI_CONVENTIONAL_MEMORY to consider the soft reserved
> > > case.
> > >
> > > Cc: <x86@xxxxxxxxxx>
> > > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > > Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
> > > Cc: Andy Shevchenko <andy@xxxxxxxxxxxxx>
> > > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> > > Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> >
> > Hi Dan,
> >
> > I understand that non-x86 may be out of scope for you, but this patch
> > makes changes to x86 and generic code at the same time without regard
> > for other architectures.
>
> Yes, that did give me pause.
>
> > I'd prefer it if we could cover ARM cleanly as well right at the start.
>
> Let's do it.
>
> >
> > The first step would be to split out the EFI stub changes (i.e., to
> > avoid allocating memory from EFI_MEMORY_SP regions) and the EFI core
> > changes from the other changes. Then, I would like to ask for your
> > help to get the arm64 part implemented where EFI_MEMORY_SP memory gets
> > registered/reserved in a way that allows the HMAT code (which should
> > be arch agnostic) to operate in the same way as it does on x86. Would
> > it be enough to simply memblock_reserve() it and insert the iomem
> > resource with the soft_reserved attribute?
> >
> > Some more comments below.
> >
> > > ---
> > > Documentation/admin-guide/kernel-parameters.txt | 19 +++++++--
> > > arch/x86/Kconfig | 21 +++++++++
> > > arch/x86/boot/compressed/eboot.c | 7 +++
> > > arch/x86/boot/compressed/kaslr.c | 4 ++
> > > arch/x86/include/asm/e820/types.h | 8 ++++
> > > arch/x86/include/asm/efi-stub.h | 11 +++++
> > > arch/x86/kernel/e820.c | 12 +++++
> > > arch/x86/platform/efi/efi.c | 51 +++++++++++++++++++++--
> > > drivers/firmware/efi/efi.c | 3 +
> > > drivers/firmware/efi/libstub/efi-stub-helper.c | 12 +++++
> > > include/linux/efi.h | 1
> > > include/linux/ioport.h | 1
> > > 12 files changed, 139 insertions(+), 11 deletions(-)
> > > create mode 100644 arch/x86/include/asm/efi-stub.h
> > >
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 1c67acd1df65..dd28f0726309 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -1152,7 +1152,8 @@
> > > Format: {"off" | "on" | "skip[mbr]"}
> > >
> > > efi= [EFI]
> > > - Format: { "old_map", "nochunk", "noruntime", "debug" }
> > > + Format: { "old_map", "nochunk", "noruntime", "debug",
> > > + "nosoftreserve" }
> > > old_map [X86-64]: switch to the old ioremap-based EFI
> > > runtime services mapping. 32-bit still uses this one by
> > > default.
> > > @@ -1161,6 +1162,12 @@
> > > firmware implementations.
> > > noruntime : disable EFI runtime services support
> > > debug: enable misc debug output
> > > + nosoftreserve: The EFI_MEMORY_SP (Specific Purpose)
> > > + attribute may cause the kernel to reserve the
> > > + memory range for a memory mapping driver to
> > > + claim. Specify efi=nosoftreserve to disable this
> > > + reservation and treat the memory by its base type
> > > + (i.e. EFI_CONVENTIONAL_MEMORY / "System RAM").
> > >
> > > efi_no_storage_paranoia [EFI; X86]
> > > Using this parameter you can use more than 50% of
> > > @@ -1173,15 +1180,21 @@
> > > updating original EFI memory map.
> > > Region of memory which aa attribute is added to is
> > > from ss to ss+nn.
> > > +
> > > If efi_fake_mem=2G@4G:0x10000,2G@0x10a0000000:0x10000
> > > is specified, EFI_MEMORY_MORE_RELIABLE(0x10000)
> > > attribute is added to range 0x100000000-0x180000000 and
> > > 0x10a0000000-0x1120000000.
> > >
> > > + If efi_fake_mem=8G@9G:0x40000 is specified, the
> > > + EFI_MEMORY_SP(0x40000) attribute is added to
> > > + range 0x240000000-0x43fffffff.
> > > +
> > > Using this parameter you can do debugging of EFI memmap
> > > - related feature. For example, you can do debugging of
> > > + related features. For example, you can do debugging of
> > > Address Range Mirroring feature even if your box
> > > - doesn't support it.
> > > + doesn't support it, or mark specific memory as
> > > + "soft reserved".
> > >
> > > efivar_ssdt= [EFI; X86] Name of an EFI variable that contains an SSDT
> > > that is to be dynamically loaded by Linux. If there are
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 4195f44c6a09..bced13503bb1 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -1981,6 +1981,27 @@ config EFI_MIXED
> > >
> > > If unsure, say N.
> > >
> > > +config EFI_SOFT_RESERVE
> > > + bool "Reserve EFI Specific Purpose Memory"
> > > + depends on EFI && ACPI_HMAT
> > > + default ACPI_HMAT
> > > + ---help---
> > > + On systems that have mixed performance classes of memory EFI
> > > + may indicate specific purpose memory with an attribute (See
> > > + EFI_MEMORY_SP in UEFI 2.8). A memory range tagged with this
> > > + attribute may have unique performance characteristics compared
> > > + to the system's general purpose "System RAM" pool. On the
> > > + expectation that such memory has application specific usage,
> > > + and its base EFI memory type is "conventional" answer Y to
> > > + arrange for the kernel to reserve it as a "Soft Reserved"
> > > + resource, and set aside for direct-access (device-dax) by
> > > + default. The memory range can later be optionally assigned to
> > > + the page allocator by system administrator policy via the
> > > + device-dax kmem facility. Say N to have the kernel treat this
> > > + memory as "System RAM" by default.
> > > +
> > > + If unsure, say Y.
> > > +
> >
> > This should be in generic code.
>
> Agree.
>
> >
> > > config SECCOMP
> > > def_bool y
> > > prompt "Enable seccomp to safely compute untrusted bytecode"
> > > diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
> > > index d6662fdef300..f2dc5896d770 100644
> > > --- a/arch/x86/boot/compressed/eboot.c
> > > +++ b/arch/x86/boot/compressed/eboot.c
> > > @@ -10,6 +10,7 @@
> > > #include <linux/pci.h>
> > >
> > > #include <asm/efi.h>
> > > +#include <asm/efi-stub.h>
> > > #include <asm/e820/types.h>
> > > #include <asm/setup.h>
> > > #include <asm/desc.h>
> > > @@ -553,7 +554,11 @@ setup_e820(struct boot_params *params, struct setup_data *e820ext, u32 e820ext_s
> > > case EFI_BOOT_SERVICES_CODE:
> > > case EFI_BOOT_SERVICES_DATA:
> > > case EFI_CONVENTIONAL_MEMORY:
> > > - e820_type = E820_TYPE_RAM;
> > > + if (!efi_nosoftreserve
> > > + && (d->attribute & EFI_MEMORY_SP))
> > > + e820_type = E820_TYPE_SOFT_RESERVED;
> > > + else
> > > + e820_type = E820_TYPE_RAM;
> > > break;
> > >
> > > case EFI_ACPI_MEMORY_NVS:
> > > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > > index 2e53c056ba20..093e84e28b7a 100644
> > > --- a/arch/x86/boot/compressed/kaslr.c
> > > +++ b/arch/x86/boot/compressed/kaslr.c
> > > @@ -38,6 +38,7 @@
> > > #include <linux/efi.h>
> > > #include <generated/utsrelease.h>
> > > #include <asm/efi.h>
> > > +#include <asm/efi-stub.h>
> > >
> > > /* Macros used by the included decompressor code below. */
> > > #define STATIC
> > > @@ -760,6 +761,9 @@ process_efi_entries(unsigned long minimum, unsigned long image_size)
> > > if (md->type != EFI_CONVENTIONAL_MEMORY)
> > > continue;
> > >
> > > + if (!efi_nosoftreserve && (md->attribute & EFI_MEMORY_SP))
> > > + continue;
> > > +
> > > if (efi_mirror_found &&
> > > !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
> > > continue;
> > > diff --git a/arch/x86/include/asm/e820/types.h b/arch/x86/include/asm/e820/types.h
> > > index c3aa4b5e49e2..314f75d886d0 100644
> > > --- a/arch/x86/include/asm/e820/types.h
> > > +++ b/arch/x86/include/asm/e820/types.h
> > > @@ -28,6 +28,14 @@ enum e820_type {
> > > */
> > > E820_TYPE_PRAM = 12,
> > >
> > > + /*
> > > + * Special-purpose memory is indicated to the system via the
> > > + * EFI_MEMORY_SP attribute. Define an e820 translation of this
> > > + * memory type for the purpose of reserving this range and
> > > + * marking it with the IORES_DESC_SOFT_RESERVED designation.
> > > + */
> > > + E820_TYPE_SOFT_RESERVED = 0xefffffff,
> > > +
> > > /*
> > > * Reserved RAM used by the kernel itself if
> > > * CONFIG_INTEL_TXT=y is enabled, memory of this type
> > > diff --git a/arch/x86/include/asm/efi-stub.h b/arch/x86/include/asm/efi-stub.h
> > > new file mode 100644
> > > index 000000000000..16ebd036387b
> > > --- /dev/null
> > > +++ b/arch/x86/include/asm/efi-stub.h
> > > @@ -0,0 +1,11 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +#ifndef _X86_EFI_STUB_H_
> > > +#define _X86_EFI_STUB_H_
> > > +
> > > +#ifdef CONFIG_EFI_STUB
> > > +extern bool efi_nosoftreserve;
> > > +#else
> > > +#define efi_nosoftreserve (1)
> > > +#endif
> > > +
> > > +#endif /* _X86_EFI_STUB_H_ */
> >
> > Please put this in generic code as well (but you need a function not a
> > variable - see below)
> >
> > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > index 7da2bcd2b8eb..9976106b57ec 100644
> > > --- a/arch/x86/kernel/e820.c
> > > +++ b/arch/x86/kernel/e820.c
> > > @@ -190,6 +190,7 @@ static void __init e820_print_type(enum e820_type type)
> > > case E820_TYPE_RAM: /* Fall through: */
> > > case E820_TYPE_RESERVED_KERN: pr_cont("usable"); break;
> > > case E820_TYPE_RESERVED: pr_cont("reserved"); break;
> > > + case E820_TYPE_SOFT_RESERVED: pr_cont("soft reserved"); break;
> > > case E820_TYPE_ACPI: pr_cont("ACPI data"); break;
> > > case E820_TYPE_NVS: pr_cont("ACPI NVS"); break;
> > > case E820_TYPE_UNUSABLE: pr_cont("unusable"); break;
> > > @@ -1037,6 +1038,7 @@ static const char *__init e820_type_to_string(struct e820_entry *entry)
> > > case E820_TYPE_PRAM: return "Persistent Memory (legacy)";
> > > case E820_TYPE_PMEM: return "Persistent Memory";
> > > case E820_TYPE_RESERVED: return "Reserved";
> > > + case E820_TYPE_SOFT_RESERVED: return "Soft Reserved";
> > > default: return "Unknown E820 type";
> > > }
> > > }
> > > @@ -1052,6 +1054,7 @@ static unsigned long __init e820_type_to_iomem_type(struct e820_entry *entry)
> > > case E820_TYPE_PRAM: /* Fall-through: */
> > > case E820_TYPE_PMEM: /* Fall-through: */
> > > case E820_TYPE_RESERVED: /* Fall-through: */
> > > + case E820_TYPE_SOFT_RESERVED: /* Fall-through: */
> > > default: return IORESOURCE_MEM;
> > > }
> > > }
> > > @@ -1064,6 +1067,7 @@ static unsigned long __init e820_type_to_iores_desc(struct e820_entry *entry)
> > > case E820_TYPE_PMEM: return IORES_DESC_PERSISTENT_MEMORY;
> > > case E820_TYPE_PRAM: return IORES_DESC_PERSISTENT_MEMORY_LEGACY;
> > > case E820_TYPE_RESERVED: return IORES_DESC_RESERVED;
> > > + case E820_TYPE_SOFT_RESERVED: return IORES_DESC_SOFT_RESERVED;
> > > case E820_TYPE_RESERVED_KERN: /* Fall-through: */
> > > case E820_TYPE_RAM: /* Fall-through: */
> > > case E820_TYPE_UNUSABLE: /* Fall-through: */
> > > @@ -1078,11 +1082,12 @@ static bool __init do_mark_busy(enum e820_type type, struct resource *res)
> > > return true;
> > >
> > > /*
> > > - * Treat persistent memory like device memory, i.e. reserve it
> > > - * for exclusive use of a driver
> > > + * Treat persistent memory and other special memory ranges like
> > > + * device memory, i.e. reserve it for exclusive use of a driver
> > > */
> > > switch (type) {
> > > case E820_TYPE_RESERVED:
> > > + case E820_TYPE_SOFT_RESERVED:
> > > case E820_TYPE_PRAM:
> > > case E820_TYPE_PMEM:
> > > return false;
> > > @@ -1285,6 +1290,9 @@ void __init e820__memblock_setup(void)
> > > if (end != (resource_size_t)end)
> > > continue;
> > >
> > > + if (entry->type == E820_TYPE_SOFT_RESERVED)
> > > + memblock_reserve(entry->addr, entry->size);
> > > +
> > > if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
> > > continue;
> > >
> > > diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> > > index 0bb58eb33ca0..9cfb7f1cf25d 100644
> > > --- a/arch/x86/platform/efi/efi.c
> > > +++ b/arch/x86/platform/efi/efi.c
> > > @@ -151,10 +151,18 @@ void __init efi_find_mirror(void)
> > > * more than the max 128 entries that can fit in the e820 legacy
> > > * (zeropage) memory map.
> > > */
> > > +enum add_efi_mode {
> > > + ADD_EFI_ALL,
> > > + ADD_EFI_SOFT_RESERVED,
> > > +};
> > >
> > > -static void __init do_add_efi_memmap(void)
> > > +static void __init do_add_efi_memmap(enum add_efi_mode mode)
> > > {
> > > efi_memory_desc_t *md;
> > > + int add = 0;
> > > +
> > > + if (!efi_enabled(EFI_MEMMAP))
> > > + return;
> > >
> > > for_each_efi_memory_desc(md) {
> > > unsigned long long start = md->phys_addr;
> > > @@ -167,7 +175,10 @@ static void __init do_add_efi_memmap(void)
> > > case EFI_BOOT_SERVICES_CODE:
> > > case EFI_BOOT_SERVICES_DATA:
> > > case EFI_CONVENTIONAL_MEMORY:
> > > - if (md->attribute & EFI_MEMORY_WB)
> > > + if (efi_enabled(EFI_MEM_SOFT_RESERVE)
> > > + && (md->attribute & EFI_MEMORY_SP))
> > > + e820_type = E820_TYPE_SOFT_RESERVED;
> > > + else if (md->attribute & EFI_MEMORY_WB)
> > > e820_type = E820_TYPE_RAM;
> > > else
> > > e820_type = E820_TYPE_RESERVED;
> > > @@ -193,9 +204,17 @@ static void __init do_add_efi_memmap(void)
> > > e820_type = E820_TYPE_RESERVED;
> > > break;
> > > }
> > > +
> > > + if (e820_type == E820_TYPE_SOFT_RESERVED)
> > > + /* always add E820_TYPE_SOFT_RESERVED */;
> > > + else if (mode == ADD_EFI_SOFT_RESERVED)
> > > + continue;
> > > +
> > > + add++;
> > > e820__range_add(start, size, e820_type);
> > > }
> > > - e820__update_table(e820_table);
> > > + if (add)
> > > + e820__update_table(e820_table);
> > > }
> > >
> > > int __init efi_memblock_x86_reserve_range(void)
> > > @@ -227,8 +246,18 @@ int __init efi_memblock_x86_reserve_range(void)
> > > if (rv)
> > > return rv;
> > >
> > > - if (add_efi_memmap)
> > > - do_add_efi_memmap();
> > > + if (add_efi_memmap) {
> > > + do_add_efi_memmap(ADD_EFI_ALL);
> > > + } else {
> > > + /*
> > > + * Given add_efi_memmap defaults to 0 and there there is no e820
> > > + * mechanism for soft-reserved memory. Explicitly scan for
> > > + * soft-reserved memory. Otherwise, the mechanism to disable the
> > > + * kernel's consideration of EFI_MEMORY_SP is the
> > > + * efi=nosoftreserve option.
> > > + */
> > > + do_add_efi_memmap(ADD_EFI_SOFT_RESERVED);
> > > + }
> > >
> > > WARN(efi.memmap.desc_version != 1,
> > > "Unexpected EFI_MEMORY_DESCRIPTOR version %ld",
> > > @@ -781,6 +810,15 @@ static bool should_map_region(efi_memory_desc_t *md)
> > > if (IS_ENABLED(CONFIG_X86_32))
> > > return false;
> > >
> > > + /*
> > > + * EFI specific purpose memory may be reserved by default
> > > + * depending on kernel config and boot options.
> > > + */
> > > + if (md->type == EFI_CONVENTIONAL_MEMORY
> > > + && efi_enabled(EFI_MEM_SOFT_RESERVE)
> > > + && (md->attribute & EFI_MEMORY_SP))
> > > + return false;
> > > +
> > > /*
> > > * Map all of RAM so that we can access arguments in the 1:1
> > > * mapping when making EFI runtime calls.
> > > @@ -1072,6 +1110,9 @@ static int __init arch_parse_efi_cmdline(char *str)
> > > if (parse_option_str(str, "old_map"))
> > > set_bit(EFI_OLD_MEMMAP, &efi.flags);
> > >
> > > + if (parse_option_str(str, "nosoftreserve"))
> > > + clear_bit(EFI_MEM_SOFT_RESERVE, &efi.flags);
> > > +
> >
> > Can we move this to the generic efi= handling code?
>
> To parse_efi_cmdline() in drivers/fimrware/efi.c? Sure.
>
> >
> > > return 0;
> > > }
> > > early_param("efi", arch_parse_efi_cmdline);
> > > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > > index 363bb9d00fa5..6d54d5c74347 100644
> > > --- a/drivers/firmware/efi/efi.c
> > > +++ b/drivers/firmware/efi/efi.c
> > > @@ -52,6 +52,9 @@ struct efi __read_mostly efi = {
> > > .tpm_log = EFI_INVALID_TABLE_ADDR,
> > > .tpm_final_log = EFI_INVALID_TABLE_ADDR,
> > > .mem_reserve = EFI_INVALID_TABLE_ADDR,
> > > +#ifdef CONFIG_EFI_SOFT_RESERVE
> > > + .flags = 1UL << EFI_MEM_SOFT_RESERVE,
> > > +#endif
> > > };
> > > EXPORT_SYMBOL(efi);
> > >
> >
> > I'd prefer it if we could call this EFI_MEM_NO_SOFT_RESERVE instead,
> > and invert the meaning of the bit.
>
> ...but that would mean repeat occurrences of
> "!efi_enabled(EFI_MEM_NO_SOFT_RESERVE)", doesn't the double negative
> seem less readable to you?
>
One the one hand, yes. On the other hand, it is the only flag whose
default is 'enabled' which is also less than ideal.
> >
> > > diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> > > index 3caae7f2cf56..35ee98a2c00c 100644
> > > --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> > > +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> > > @@ -28,6 +28,7 @@
> > > #define EFI_READ_CHUNK_SIZE (1024 * 1024)
> > >
> > > static unsigned long __chunk_size = EFI_READ_CHUNK_SIZE;
> > > +bool efi_nosoftreserve;
> > >
> >
> > This needs a getter function if you want to access it from other
> > compilation units. This has to do with how the early relocation code
> > handles data symbol references. Please refer to nokaslr() for an
> > example.
>
> Ah, does that mean that the efi_nosoftreserve global variable
> instances in different compilation units are effectively static the
> way I currently have them defined?
No, the problem had to do with relocation of GOT entries on some x86
builds. Then, things got more complicated when I added the 32-bit ARM
port, which puts other constraints related to how symbols are placed
in the binary.
So please duplicate the pattern with the static variable and the
__pure setter, which has proven to be the most robust way to expose
variables to other compilation units in the stub.
