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Subject: Re: [edk2-devel] [RFC PATCH v2 04/44] OvmfPkg/ResetVector: Add support for a 32-bit SEV check
To: "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
 "devel@edk2.groups.io" <devel@edk2.groups.io>
Cc: Jordan Justen <jordan.l.justen@intel.com>,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>,
 Michael D Kinney <michael.d.kinney@intel.com>,
 Liming Gao <liming.gao@intel.com>, Eric Dong <eric.dong@intel.com>,
 Ray Ni <ray.ni@intel.com>, "Singh, Brijesh" <brijesh.singh@amd.com>
References: <cover.1568922728.git.thomas.lendacky@amd.com>
 <54ebf48fe05c20a1181a3dc90496e4835912ebf2.1568922728.git.thomas.lendacky@amd.com>
 <dbce9d65-5144-7a06-2f0b-9f6dad6eaeea@redhat.com>
 <4dda0b6e-c464-ef55-2745-ad6beabb98b7@amd.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <e8a8e21e-4045-1b2b-f959-13fbe00132d9@redhat.com>
Date: Mon, 30 Sep 2019 21:29:12 +0200
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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