linux-kernel - Re: [PATCH v11 18/20] x86/sev: Mark Secure TSC as reliable clocksource

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Message-ID: <55705fb4-65ed-5fea-8fec-36309ef0d523@amd.com>
Date: Fri, 13 Sep 2024 11:59:21 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Nikunj A Dadhania <nikunj@....com>, linux-kernel@...r.kernel.org,
 bp@...en8.de, x86@...nel.org, kvm@...r.kernel.org
Cc: mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
 pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v11 18/20] x86/sev: Mark Secure TSC as reliable
 clocksource

On 7/31/24 10:08, Nikunj A Dadhania wrote:
> In SNP guest environment with Secure TSC enabled, unlike other clock
> sources (such as HPET, ACPI timer, APIC, etc.), the RDTSC instruction is
> handled without causing a VM exit, resulting in minimal overhead and
> jitters. Hence, mark Secure TSC as the only reliable clock source,
> bypassing unstable calibration.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> Tested-by: Peter Gonda <pgonda@...gle.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> ---
>  arch/x86/mm/mem_encrypt_amd.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
> index 86a476a426c2..e9fb5f24703a 100644
> --- a/arch/x86/mm/mem_encrypt_amd.c
> +++ b/arch/x86/mm/mem_encrypt_amd.c
> @@ -516,6 +516,10 @@ 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);
>  }
>  
>  void __init mem_encrypt_free_decrypted_mem(void)