linux-kernel - Re: [PATCH Part2 v6 05/49] x86/sev: Add RMP entry lookup helpers

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Date:   Fri, 22 Jul 2022 19:04:23 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Kalra, Ashish" <Ashish.Kalra@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
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Subject: Re: [PATCH Part2 v6 05/49] x86/sev: Add RMP entry lookup helpers

On Fri, Jul 22, 2022, Borislav Petkov wrote:
> On Thu, Jun 23, 2022 at 10:43:40PM +0000, Kalra, Ashish wrote:
> > Yes, that's a nice way to hide it from the rest of the kernel which
> > does not require access to this structure anyway, in essence, it
> > becomes a private structure.
> 
> So this whole discussion whether there should be a model check or not
> in case a new RMP format gets added in the future is moot - when a new
> model format comes along, *then* the distinction should be done and
> added in code - not earlier.

I disagree.  Running an old kernel on new hardware with a different RMP layout
should refuse to use SNP, not read/write garbage and likely corrupt the RMP and/or
host memory.

And IMO, hiding the non-architectural RMP format in SNP-specific code so that we
don't have to churn a bunch of call sites that don't _need_ access to the raw RMP
format is a good idea regardless of whether we want to be optimistic or pessimistic
about future formats.

> This is nothing else but normal CPU enablement work - it should be done
> when it is really needed.
> 
> Because the opposite can happen: you can add a model check which
> excludes future model X, future model X comes along but does *not*
> change the RMP format and then you're going to have to relax that model
> check again to fix SNP on the new model X.
> 
> So pls add the model checks only when really needed.
> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette