Re: [edk2-devel] [RFC PATCH v2 38/44] UefiCpuPkg: Allow AP booting under SEV-ES

["Liming Gao" <liming.gao@intel.com>]

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Andrew:
  Can you give more detail on how to update nasm source code to put the 64bit absolute address from .text section to .data section? I will verify it. Now, the patching way doesn't support X64 SEC/PEI. This is a gab in XCODE tool chain.

Thanks
Liming
From: devel@edk2.groups.io [mailto:devel@edk2.groups.io] On Behalf Of Andrew Fish via Groups.Io
Sent: Saturday, October 12, 2019 2:43 PM
To: devel@edk2.groups.io; Laszlo Ersek <lersek@redhat.com>
Cc: Lendacky, Thomas <Thomas.Lendacky@amd.com>; Justen, Jordan L <jordan.l.justen@intel.com>; Ard Biesheuvel <ard.biesheuvel@linaro.org>; Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>; Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Singh, Brijesh <brijesh.singh@amd.com>; Philippe Mathieu-Daudé <philmd@redhat.com>
Subject: Re: [edk2-devel] [RFC PATCH v2 38/44] UefiCpuPkg: Allow AP booting under SEV-ES

Laszlo,

For 2) this  is very unfortunate. I think the root cause is for those of us who work on x86 hardware day to day we get programed that SEC/PEI is IA32 and DXE is X64, and this can lead to some unfortunate coding outcomes.

I'm guessing this code probably got ported from the DXE CPU driver or some other place that had no XIP assumptions. One option vs. patching is putting the relocations in the .data section. The only issue with that could be the need to align sections on page boundaries and that may take up too much space in XIP code. Perhaps we could only require the .data section relocations for XCODE, and map them to .text for the other toolchain?

Thanks,

Andrew Fish


On Oct 11, 2019, at 1:56 AM, Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>> wrote:

On 10/11/19 01:17, Lendacky, Thomas wrote:

On 10/3/19 10:12 AM, Tom Lendacky wrote:



On 10/3/19 5:32 AM, Laszlo Ersek wrote:

On 10/03/19 12:12, Laszlo Ersek wrote:


 UINT32   ApEntryPoint;
 EFI_GUID SevEsFooterGuid;
 UINT16   Size;

It's probably better to reverse the order of "Size" and
"SevEsFooterGuid", like this:

 UINT32   ApEntryPoint;
 UINT16   Size;
 EFI_GUID SevEsFooterGuid;

because then even the "Size" field can be changed (or resized), as a
function of the footer GUID.

Cool, I'll look into doing this and see how it works out.

Just an update on this idea. This has worked out well, but has a couple of
caveats. Removing the Qemu change to make the flash mapped read-only in
the nested page tables, caused the following:

1. QemuFlashDetected() will attempt to detect how the flash memory device
  behaves. Because it is marked as read-only by the hypervisor, writing
  to the area results in a #NPF for the write-fault. With SEV-ES,
  emulation of the instruction can't be performed (can't read guest
  memory and not provided the faulting instruction bytes), so the vCPU is
  just restarted. This results in an infinite #NPF occurring.

  The solution here was to check for SEV-ES being enabled and just return
  false from QemuFlashDetected(). Any downfalls to doing that?

Short-circuiting QemuFlashDetected() on SEV-ES seems appropriate.

However, I don't understand why you return FALSE in that case. You
should return TRUE. If QemuFlashDetected() returns FALSE, then the UEFI
variable store will not be backed by the real pflash chip, it will be
emulated with an \NvVars file on the EFI system partition. That
emulation should really not be used nowadays.

So IMO the right approach here is:
- declare that SEV-ES only targets the "two pflash chips" setup
- return TRUE from QemuFlashDetected() when SEV-ES is on.



2. Commit 2db0ccc2d7fe ("UefiCpuPkg: Update CpuExceptionHandlerLib pass
  XCODE5 tool chain") causes a similar situation to #1. It attempts to do
  some address fixups and write to the flash device.

That's... stunning.

Commit 2db0ccc2d7fe changes the file

 UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm

such that it does in-place binary patching.

This source file is referenced from:

 UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf

as well. Note "SecPei".

That makes the commit buggy, to my eyes, regardless of SEV-ES. Because:

The binary patching appears to occur in the SEC phase as well, i.e. at a
time when the exception handler is located in flash. That's incorrect on
physical hardware too.

Upon re-reading <https://bugzilla.tianocore.org/show_bug.cgi?id=849>,
this commit worked around an XCODE toolchain bug.

Unfortunately, the workaround is not suitable for the SEC phase. (Also
not suitable for the PEI phase, for such PEIMs that still execute from
flash.)

Please open a new bug for UefiCpuPkg in the TianoCore Bugzilla,
reference BZ#849 in the See Also field, and please also make the new bug
block BZ#2198.

(I'll comment on this issue in a different thread too; I'll CC you on it.)


  Reverting that commit fixes the issue. I don't think that will be an
  acceptable solution, though, so need to think about what to do here.

After those two changes, the above method works well.

I'm happy to hear!

Thanks,
Laszlo





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