linux-kernel - Re: [PATCH] x86/sev: Improve handling of writes to intercepted GUEST_TSC

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Message-ID: <20250715084357.GCaHYUzeqvBxJyGVsg@fat_crate.local>
Date: Tue, 15 Jul 2025 10:43:57 +0200
From: Borislav Petkov <bp@...en8.de>
To: Nikunj A Dadhania <nikunj@....com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, dave.hansen@...ux.intel.com,
	santosh.shukla@....com
Subject: Re: [PATCH] x86/sev: Improve handling of writes to intercepted
 GUEST_TSC_FREQ

On Tue, Jul 15, 2025 at 08:37:38AM +0000, Nikunj A Dadhania wrote:
>   Currently, when a Secure TSC enabled SNP guest attempts to write to
>   the intercepted GUEST_TSC_FREQ MSR (a read-only MSR), the guest kernel
>   #VC handler terminates the SNP guest by returning ES_VMM_ERROR. This
>   response incorrectly implies a VMM configuration error, when in fact
>   it's a valid VMM configuration to intercept writes to read-only MSRs,

Not only valid - it is the usual thing the HV does with MSRs IMHO.

>   unless explicitly documented.
> 
>   Modify the intercepted GUEST_TSC_FREQ MSR #VC handler to ignore writes
>   instead of terminating the guest. Since GUEST_TSC_FREQ is a guest-only
>   MSR, ignoring writes directly (rather than forwarding to the VMM and
>   handling the resulting #GP) eliminates a round trip to the VMM.

Probably.

But I think the main point here is that this is the default action the HV
does.

> Add a
>   WARN_ONCE to log the incident, as well-behaved guest kernels should
>   never attempt to write to this read-only MSR.
> 
>   However, continue to terminate the guest(via ES_VMM_ERROR) when

ES_EXCEPTION

>   reading from intercepted GUEST_TSC_FREQ MSR with Secure TSC enabled,
>   as intercepted reads indicate an improper VMM configuration for Secure
>   TSC enabled SNP guests.

It is getting close to the gist of what we talked yesterday tho.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette