Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper

From: Borislav Petkov
Date: Thu Sep 02 2021 - 07:26:34 EST

On Mon, Aug 30, 2021 at 10:07:39AM -0500, Brijesh Singh wrote:
> The SNP firmware spec says that counter must begin with the 1.

So put that in the comment and explain what 0 is: magic or invalid or
whatnot and why is that so and that it is spec-ed this way, etc.

Just having it there without a reasoning makes one wonder whether that's
some arbitrary limitation or so.

> During the GHCB writing the seqno use to be 32-bit value and hence the GHCB
> spec choose the 32-bit value but recently the SNP firmware changed it from
> the 32 to 64. So, now we are left with the option of limiting the sequence
> number to 32-bit. If we go beyond 32-bit then all we can do is fail the
> call. If we pass the value of zero then FW will fail the call.

That sounds weird again. So make it 64-bit like the FW and fix the spec.

> I just choose the smaller name but I have no issues matching with the spec.
> Also those keys does not have anything to do with the VMPL level. The
> secrets page provides 4 different keys and they are referred as vmpck0..3
> and each of them have a sequence numbers associated with it.
> In GHCB v3 we probably need to rework the structure name.

You can point to the spec section so that readers can find the struct
layout there.