linux-kernel - Re: [PATCH v5 12/14] x86/kvmclock: Skip kvmclock when Secure TSC is available

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Date:   Mon, 30 Oct 2023 16:00:03 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Nikunj A Dadhania <nikunj@....com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com, dionnaglaze@...gle.com,
        pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v5 12/14] x86/kvmclock: Skip kvmclock when Secure TSC is
 available

On 10/30/23 01:36, Nikunj A Dadhania wrote:
> For AMD SNP guests having Secure TSC enabled, skip using the kvmclock.
> The guest kernel will fallback and use Secure TSC based clocksource.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
>   arch/x86/kernel/kvmclock.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index fb8f52149be9..779e7311fa6f 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -288,7 +288,7 @@ void __init kvmclock_init(void)
>   {
>   	u8 flags;
>   
> -	if (!kvm_para_available() || !kvmclock)
> +	if (!kvm_para_available() || !kvmclock || cc_platform_has(CC_ATTR_GUEST_SECURE_TSC))

And is setting X86_FEATURE_TSC_RELIABLE, as Dave Hansen suggests, enough
to prevent usage of kvmclock?

There was a discussion here:
  https://lore.kernel.org/lkml/20230808162320.27297-1-kirill.shutemov@linux.intel.com/

Thanks,
Tom

>   		return;
>   
>   	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2)) {