linux-kernel - Re: [PATCH v5 13/14] x86/tsc: Mark Secure TSC as reliable clocksource

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Date:   Thu, 2 Nov 2023 13:33:06 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     "Nikunj A. Dadhania" <nikunj@....com>
Cc:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org,
        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 13/14] x86/tsc: Mark Secure TSC as reliable clocksource

On Thu, Nov 02, 2023 at 11:23:34AM +0530, Nikunj A. Dadhania wrote:
> On 10/30/2023 10:48 PM, Dave Hansen wrote:
> > On 10/29/23 23:36, Nikunj A Dadhania wrote:
> > ...
> >> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> >> index 15f97c0abc9d..b0a8546d3703 100644
> >> --- a/arch/x86/kernel/tsc.c
> >> +++ b/arch/x86/kernel/tsc.c
> >> @@ -1241,7 +1241,7 @@ static void __init check_system_tsc_reliable(void)
> >>  			tsc_clocksource_reliable = 1;
> >>  	}
> >>  #endif
> >> -	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
> >> +	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE) || cc_platform_has(CC_ATTR_GUEST_SECURE_TSC))
> >>  		tsc_clocksource_reliable = 1;
> > 
> > Why can't you just set X86_FEATURE_TSC_RELIABLE?
> 
> Last time when I tried, I had removed my kvmclock changes and I had set
> the X86_FEATURE_TSC_RELIABLE similar to Kirill's patch[1], this did not
> select the SecureTSC.
> 
> Let me try setting X86_FEATURE_TSC_RELIABLE and retaining my patch for
> skipping kvmclock.

kvmclock lowers its rating if TSC is good enough:

	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
	    !check_tsc_unstable())
		kvm_clock.rating = 299;

Does your TSC meet the requirements?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov