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

["Andrew Fish" <afish@apple.com>]

> On Oct 10, 2019, at 4:17 PM, Lendacky, Thomas <thomas.lendacky@amd.com> 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?
> 
> 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.
> 
>   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.
> 

Did you fill a bugzilla for 2)?

Thanks,

Andrew Fish

> After those two changes, the above method works well.
> 
> Thanks,
> Tom
> 
>> 
>> Thanks!
>> Tom
>> 
>>> 
>>> Thanks
>>> Laszlo
>>> 
> 
> 
>