linux-kernel - Re: [PATCH v2] x86, clock: Fix kvm guest tsc initialization

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Date:   Thu, 8 Sep 2016 16:42:46 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Len Brown <len.brown@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>,
        David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v2] x86, clock: Fix kvm guest tsc initialization



On 08/09/2016 16:15, Prarit Bhargava wrote:
> When booting a kvm guest on AMD with the latest kernel the following
> messages are displayed in the boot log:
> 
>  tsc: Unable to calibrate against PIT
>  tsc: HPET/PMTIMER calibration failed
> 
> aa297292d708 ("x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID")
> introduced a change to account for a difference in cpu and tsc frequencies for
> Intel SKL processors. Before this change the native tsc set
> x86_platform.calibrate_tsc to native_calibrate_tsc() which is a hardware
> calibration of the tsc, and in tsc_init() executed
> 
> 	tsc_khz = x86_platform.calibrate_tsc();
> 	cpu_khz = tsc_khz;
> 
> The kvm code changed x86_platform.calibrate_tsc to kvm_get_tsc_khz() and
> executed the same tsc_init() function.  This meant that KVM guests did not
> execute the native hardware calibration function.
> 
> After aa297292d708, there are separate native calibrations for cpu_khz and
> tsc_khz.  The code sets x86_platform.calibrate_tsc to native_calibrate_tsc()
> which is now an Intel specific calibration function, and
> x86_platform.calibrate_cpu to native_calibrate_cpu() which is the "old"
> native_calibrate_tsc() function (ie, the native hardware calibration
> function).
> 
> tsc_init() now does
> 
> 	cpu_khz = x86_platform.calibrate_cpu();
> 	tsc_khz = x86_platform.calibrate_tsc();
> 	if (tsc_khz == 0)
> 		tsc_khz = cpu_khz;
> 	else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
> 		cpu_khz = tsc_khz;
> 
> The kvm code should not call the hardware initialization in
> native_calibrate_cpu(), as it isn't applicable for kvm and it didn't do that
> prior to aa297292d708.
> 
> This patch resolves this issue by setting x86_platform.calibrate_cpu to
> kvm_get_tsc_khz().
> 
> v2: I had originally set x86_platform.calibrate_cpu to
> cpu_khz_from_cpuid(), however, pbonzini pointed out that the CPUID leaf
> in that function is not available in KVM.  I have changed the function
> pointer to kvm_get_tsc_khz().
> 
> Fixes: aa297292d708 ("x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID")
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: Len Brown <len.brown@...el.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Cc: "Christopher S. Hall" <christopher.s.hall@...el.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> Cc: kvm@...r.kernel.org
> ---
>  arch/x86/kernel/kvmclock.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 1d39bfbd26bb..3692249a70f1 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -289,6 +289,7 @@ void __init kvmclock_init(void)
>  	put_cpu();
>  
>  	x86_platform.calibrate_tsc = kvm_get_tsc_khz;
> +	x86_platform.calibrate_cpu = kvm_get_tsc_khz;
>  	x86_platform.get_wallclock = kvm_get_wallclock;
>  	x86_platform.set_wallclock = kvm_set_wallclock;
>  #ifdef CONFIG_X86_LOCAL_APIC
> 

Applied to kvm/master, thanks (commit
a4497a86fb9b855c5ac8503fdc959393b00bb643).  I'll probably send a pull
request to Linus only next week though.

Paolo