Re: [PATCH] efi: arm/arm64: allow SetVirtualAddressMap() to be omitted

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On Wed, 30 Jan 2019 at 01:06, Bjorn Andersson
<bjorn.andersson@xxxxxxxxxx> wrote:
>
> On Sat 26 Jan 02:22 PST 2019, Ard Biesheuvel wrote:
>
> > The UEFI spec revision 2.7 errata A section 8.4 has the following to
> > say about the virtual memory runtime services:
> >
> >   "This section contains function definitions for the virtual memory
> >   support that may be optionally used by an operating system at runtime.
> >   If an operating system chooses to make EFI runtime service calls in a
> >   virtual addressing mode instead of the flat physical mode, then the
> >   operating system must use the services in this section to switch the
> >   EFI runtime services from flat physical addressing to virtual
> >   addressing."
> >
> > So it is pretty clear that calling SetVirtualAddressMap() is entirely
> > optional, and so there is no point in doing so unless it achieves
> > anything useful for us.
> >
> > This is not the case for 64-bit ARM. The native mapping used by the OS
> > is arbitrarily converted into another permutation of userland addresses
> > (i.e., bits [63:48] cleared), and the runtime code could easily deal
> > with the original layout in exactly the same way as it deals with the
> > converted layout. However, due to constraints related to page size
> > differences if the OS is not running with 4k pages, and related to
> > systems that may expose the individual sections of PE/COFF runtime
> > modules as different memory regions, creating the virtual layout is a
> > bit fiddly, and requires us to sort the memory map and reason about
> > adjacent regions with identical memory types etc etc.
> >
> > So the obvious fix is to stop calling SetVirtualAddressMap() altogether
> > on arm64 systems. However, to avoid surprises, which are notoriously
> > hard to diagnose when it comes to OS<->firmware interactions, let's
> > start by making it an opt-out feature, and implement support for the
> > 'efi=novamap' kernel command line parameter on ARM and arm64 systems.
> >
> > (Note that 32-bit ARM generally does require SetVirtualAddressMap() to be
> > used, given that the physical memory map and the kernel virtual address
> > map are not guaranteed to be non-overlapping like on arm64. However,
> > having support for efi=novamap,noruntime on 32-bit ARM, combined with
> > the recently proposed support for earlycon=efi, is likely to be useful
> > to diagnose boot issues on such systems if they have no accessible serial
> > port)
> >
> > Cc: Alexander Graf <agraf@xxxxxxx>
> > Cc: Heinrich Schuchardt <xypron.glpk@xxxxxx>
> > Cc: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > Cc: Leif Lindholm <leif.lindholm@xxxxxxxxxx>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
> This makes the SDM850 bootable, thanks.
>
> Tested-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
>

Thanks Bjorn



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