Re: [PATCH 2/9] OvmfPkg/VmgExitLib: Set the SW exit fields when performing VMGEXIT

["Laszlo Ersek" <lersek@redhat.com>]

On 10/15/20 10:50, Laszlo Ersek wrote:
> On 10/15/20 10:47, Laszlo Ersek wrote:
>> On 10/10/20 18:07, Tom Lendacky wrote:
>>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>>
>>> All fields that are set in the GHCB should have their associated bit in
>>> the GHCB ValidBitmap field set. Add support to set the bits for the
>>> software exit information fields when performing a VMGEXIT (SwExitCode,
>>> SwExitInfo1, SwExitInfo2).
>>>
>>> Fixes: 61bacc0fa16f ("OvmfPkg/VmgExitLib: Implement library support for VmgExitLib in OVMF")
>>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>>> Cc: Laszlo Ersek <lersek@redhat.com>
>>> Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
>>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>>> Cc: Brijesh Singh <brijesh.singh@amd.com>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>>> ---
>>>  OvmfPkg/Library/VmgExitLib/VmgExitLib.c | 30 ++++++++++++++++++++
>>>  1 file changed, 30 insertions(+)
>>>
>>> diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
>>> index 53040cc6f649..6cf649c6101b 100644
>>> --- a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
>>> +++ b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
>>> @@ -78,6 +78,32 @@ VmgExitErrorCheck (
>>>    return Status;
>>>  }
>>>  
>>> +/**
>>> +  Marks a field at the specified offset as valid in the GHCB.
>>> +
>>> +  The ValidBitmap area represents the areas of the GHCB that have been marked
>>> +  valid. Set the area of the GHCB at the specified offset as valid.
>>> +
>>> +  @param[in, out] Ghcb    Pointer to the Guest-Hypervisor Communication Block
>>> +  @param[in] Offset       Offset in the GHCB to mark valid
>>> +
>>> +**/
>>> +STATIC
>>> +VOID
>>> +GhcbSetOffsetValid (
>>> +  IN OUT GHCB               *Ghcb,
>>> +  IN     GHCB_QWORD_OFFSET  Offset
>>> +  )
>>> +{
>>> +  UINT32  OffsetIndex;
>>> +  UINT32  OffsetBit;
>>> +
>>> +  OffsetIndex = Offset / 8;
>>> +  OffsetBit   = Offset & 0x07;
>>
>> (1) I suggest improving the consistency of operators -- please either
>> use division and remainder ("Offset / 8" and "Offset % 8"), or bit shift
>> and masking ("Offset >> 3" and "Offset & 0x7")
>>
>> With that:
>>
>> Acked-by: Laszlo Ersek <lersek@redhat.com>
> 
> ... I realize I didn't make the same comment for GhcbIsRegValid(), so
> I'm taking back the above -- consistency with GhcbIsRegValid() is more
> important. No change needed here.
> 
> Acked-by: Laszlo Ersek <lersek@redhat.com>

Wow, I'm needing *a lot* of time for getting back into this. Sorry about
that. :/

So, as I'm slowly grasping the idea behind this series (--> wherever we
make a VmgExit() call, having populated some fields in the GHCB, make
sure the "valid bitmap" is set correctly too), it's becoming clear that
we need a new "VmgExitLib.h" API.

Because, (a) VmgExit() is declared in that lib class header anyway, and
the new helper function effectively helps us set up the (thick)
parameters for that call, (b) at the end of this v1 series, we have the
"valid bitmap" setting logic triplicated (not counting the assembly code
logic in patch #5):

- GhcbSetRegValid() -- existent function in
"OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c"

- GhcbSetOffsetValid() -- function basically identical to
GhcbSetRegValid(), added in this patch to
"OvmfPkg/Library/VmgExitLib/VmgExitLib.c". (This means that the same
library instance will have two copies of the same logic.)

- QemuFlashPtrWrite() -- from patch #6

So I think we should first replace the GhcbSetRegValid() function with a
public (UefiCpuPkg VmgExitLib) API called VmgSetOffsetValid(). This
would likely take two patches -- first patch, introduce the API and add
the VmgExitLibNull implementation under UefiCpuPkg; second patch, add
the real implementation, replacing GhcbSetRegValid(), under OvmfPkg.
Then, in the rest of the series, call VmgSetOffsetValid() wherever
needed (in C code, of course; in assembly, the open-coded stuff is OK I
guess).

Thanks,
Laszlo