linux-kernel - Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

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Message-ID: <20241210114357.GGZ1gpfWVLixGKXc0s@fat_crate.local>
Date: Tue, 10 Dec 2024 12:43:57 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Nikunj A. Dadhania" <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
	kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, pgonda@...gle.com, seanjc@...gle.com,
	pbonzini@...hat.com
Subject: Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure
 TSC enabled guests

On Tue, Dec 10, 2024 at 10:32:23AM +0530, Nikunj A. Dadhania wrote:
> That is the warning from the APM: 15.36.18 Secure TSC
> 
> "Guests that run with Secure TSC enabled are not expected to perform writes to
> the TSC MSR (10h). If such a write occurs, subsequent TSC values read are
> undefined."
> 
> What I make out of it is: if a write is performed to the TSC MSR, subsequent
> reads of TSC is not reliable/trusted.

Basically, what happens on baremetal too.

> Do you also want to terminate the offending guest?
> 
> ES_UNSUPPORTED return will do that.

I guess that would be too harsh. I guess a warn and a ES_OK should be fine for
now.

> This is changing the behavior for SEV-ES and SNP guests(non SECURE_TSC), TSC
> MSR reads are converted to RDTSC. This is a good optimization. But just
> wanted to bring up the subtle impact.

That RDTSC happens still in the guest, right? But in its #VC handler. Versus it
being a HV GHCB protocol call. I guess this conversion should be a separate
patch in case there's some issues like the HV intercepting RDTSC... i.e.,
VMEXIT_RDTSC.

We should probably handle that case too and then fallback to the GHCB call. Or
is there a catch 22 I'm missing here...

> Yes, I was thinking about a switch, as there will be more such instances when we
> enable newer features.

Exactly.

Thx.

-- 
Regards/Gruss,
    Boris.

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