Re: [edk2-devel] [RFC PATCH v2 04/44] OvmfPkg/ResetVector: Add support for a 32-bit SEV check

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

On 09/24/19 20:57, Lendacky, Thomas wrote:
> On 9/24/19 8:42 AM, Laszlo Ersek wrote:
>> On 09/19/19 21:52, Lendacky, Thomas wrote:

>>> +    ; Issue VMGEXIT (rep; vmmcall)
>>> +    db      0xf3
>>> +    db      0x0f
>>> +    db      0x01
>>> +    db      0xd9
>>
>> (8) Can you please file an RFE at <https://bugzilla.nasm.us/>, for
>> supporting this instruction, and add the link here, as a comment? I've
>> been fighting an uphill battle against DB-encoded instructions in edk2
>> assembly code.
> 
> Yes, let me look into that.

Actually, from peeking ahead at patch "MdePkg/BaseLib: Add support for
the VMGEXIT instruction", it looks like "rep; vmmcall" is already
understood by NASM.

Can you use that here?

... In case that sequence of mnemonics is specific to NASM's 64-bit
mode: can you bracket it with BITS 64 / BITS 32, just so we can avoid
the DBs? Something like (if necessary):

; Issue VMGEXIT
BITS 64
rep; vmmcall
BITS 32


Hmmm why don't I try this out myself...

... So, first, the semicolon (;) seems wrong in the NASM source. It
turns vmmcall into a comment, and NASM assembles only the REP prefix (to
a single 0xF3 byte).

Second, when I remove the semicolon, NASM indeed complains in 32-bit
mode "error: instruction not supported in 32-bit mode".

But the following does work:

BITS 64
rep vmmcall
BITS 32

and for it, NASM generates the bytes seen above (f3 0f 01 d9).

So I suggest:
- using this pattern in the present patch
- using this pattern in the Ia32/VmgExit.nasm source file in the
MdePkg/BaseLib patch
- removing the semicolon in the X64/VmgExit.nasm source file in the
MdePkg/BaseLib patch

Thanks!
Laszlo