On Thu, Jul 31, 2014 at 03:11:49PM +0100, Ard Biesheuvel wrote:
> There are 2 interesting pieces of information in the UEFI spec section 2.3.6
> regarding the mapping of runtime regions:
> (a) the firmware should not request a virtual mapping for configuration tables,
> even though they are marked as EfiRuntimeServicesData;
> (b) calling SetVirtualAddressMap() is optional, and it is equally appropriate to
> call Runtime Services using an identity mapping.
>
> So we can eliminate some of the complexity around UEFI Runtime Services by not
> using a virtual mapping at all, and calling the services at their physical
> address. This is especially useful under kexec, as SetVirtualAddressMap() may
> only be called once, and there is no guarantee that mappings are stable between
> different kexec'd kernels.
>
> The fallout for other in-kernel users of UEFI data structures should be
> negligible, as they cannot legally access those data structures through
> pre-existing virtual mappings anyway (point (a) above)
>
> It should also be noted that, as the kernel side of the address space (TTBR1) is
> retained, the stack and pointer function arguments remain accessible to the
> runtime service while the id mapping is active.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/efi.h | 24 ++++++++--
> arch/arm64/kernel/efi.c | 106 ++-----------------------------------------
> 2 files changed, 23 insertions(+), 107 deletions(-)
>
> diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
> index a34fd3b12e2b..d42a21e79b39 100644
> --- a/arch/arm64/include/asm/efi.h
> +++ b/arch/arm64/include/asm/efi.h
> @@ -1,8 +1,10 @@
> #ifndef _ASM_EFI_H
> #define _ASM_EFI_H
>
> +#include <asm/cacheflush.h>
> #include <asm/io.h>
> #include <asm/neon.h>
> +#include <asm/tlbflush.h>
>
> #ifdef CONFIG_EFI
> extern void efi_init(void);
> @@ -12,23 +14,37 @@ extern void efi_idmap_init(void);
> #define efi_idmap_init()
> #endif
>
> +static inline void switch_pgd(pgd_t *pgd, struct mm_struct *mm)
> +{
> + cpu_switch_mm(pgd, mm);
> + flush_tlb_all();
> + if (icache_is_aivivt())
> + __flush_icache_all();
> +}
> +
> #define efi_call_virt(f, ...) \
> ({ \
> - efi_##f##_t *__f = efi.systab->runtime->f; \
> + efi_##f##_t *__f; \
> efi_status_t __s; \
> \
> - kernel_neon_begin(); \
> + kernel_neon_begin(); /* disables preemption */ \
> + switch_pgd(idmap_pg_dir, &init_mm); \
> + __f = efi.systab->runtime->f; \
> __s = __f(__VA_ARGS__); \
> + switch_pgd(current->active_mm->pgd, current->active_mm); \
> kernel_neon_end(); \
> __s; \
> })
This scares the bejesus out of me, but I can't put my finger on exactly why.
I think it does what you intend and I can't break it myself, so it would be
really good if the EFI folks could confirm that this looks good to them.
Cheers,
Will
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