linux-kernel - Re: [PATCH v11 19/20] x86/kvmclock: Skip kvmclock when Secure TSC is available

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Message-ID: <d6be5402-5e24-0e9e-88fe-3c30fd4551d5@amd.com>
Date: Fri, 13 Sep 2024 12:19:38 -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 19/20] x86/kvmclock: Skip kvmclock when Secure TSC is
 available

On 7/31/24 10:08, Nikunj A Dadhania wrote:
> For AMD SNP guests with SecureTSC enabled, kvm-clock is being picked up
> momentarily instead of selecting more stable TSC clocksource.
> 
> [    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
> [    0.000001] kvm-clock: using sched offset of 1799357702246960 cycles
> [    0.001493] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
> [    0.006289] tsc: Detected 1996.249 MHz processor
> [    0.305123] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x398cadd9d93, max_idle_ns: 881590552906 ns
> [    1.045759] clocksource: Switched to clocksource kvm-clock
> [    1.141326] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x398cadd9d93, max_idle_ns: 881590552906 ns
> [    1.144634] clocksource: Switched to clocksource tsc
> 
> When Secure TSC is enabled, skip using the kvmclock. The guest kernel will
> fallback and use Secure TSC based clocksource.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> Tested-by: Peter Gonda <pgonda@...gle.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@....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 5b2c15214a6b..3d03b4c937b9 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -289,7 +289,7 @@ 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))
>  		return;
>  
>  	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2)) {