linux-kernel - Re: [PATCH v15 07/13] x86/sev: Mark Secure TSC as reliable clocksource

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Message-ID: <20241211203214.GDZ1n2zvfqjYj4TpzB@fat_crate.local>
Date: Wed, 11 Dec 2024 21:32:14 +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 07/13] x86/sev: Mark Secure TSC as reliable
 clocksource

On Tue, Dec 03, 2024 at 02:30:39PM +0530, Nikunj A Dadhania wrote:
> diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
> index 774f9677458f..fa0bc52ef707 100644
> --- a/arch/x86/mm/mem_encrypt_amd.c
> +++ b/arch/x86/mm/mem_encrypt_amd.c
> @@ -541,6 +541,10 @@ void __init sme_early_init(void)
>  	 * kernel mapped.
>  	 */
>  	snp_update_svsm_ca();
> +
> +	/* Mark the TSC as reliable when Secure TSC is enabled */
> +	if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
> +		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);

What happens if someone writes MSR 0x10 on some CPU and thus makes the TSCs on
the host unsynchronized and that CPU runs a SecureTSC guest?

That guest would use RDTSC and get wrong values and turn the guest into
a mess, right?

-- 
Regards/Gruss,
    Boris.

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