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

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On Sat, 26 Jan 2019 at 15:33, Heinrich Schuchardt <xypron.glpk@xxxxxx> wrote:
>
> On 1/26/19 1:34 PM, Alexander Graf wrote:
> >
> >
> >> Am 26.01.2019 um 13:28 schrieb Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>:
> >>
> >>> On Sat, 26 Jan 2019 at 13:27, Heinrich Schuchardt <xypron.glpk@xxxxxx> wrote:
> >>>
> >>>> On 1/26/19 11:22 AM, 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)
> >>>>
> >>>
> >>> NAK
> >>>
> >>
> >> Excuse me?
> >>
> >>> This patch breaks EFI booting with any known U-Boot release.
> >>>
> >>
> >> It does if you pass 'efi=novmap'. Otherwise, it works fine.
>
> It think it would be helpful to add this information to the commit message.
>

Did you read the commit message?

> If it is strictly opt-in, I have no concern.
>

Thanks.



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