linux-kernel - Re: [PATCH] x86/vmware: avoid TSC recalibration

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Date:   Tue, 05 Jan 2021 14:06:08 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Alexey Makhalov <amakhalov@...are.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, hpa@...or.com,
        bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        pv-drivers@...are.com, sdeep@...are.com
Subject: Re: [PATCH] x86/vmware: avoid TSC recalibration

Alexey Makhalov <amakhalov@...are.com> writes:

> When TSC frequency is known (retrieved from hypervisor), we should skip
> TSC refined calibration by setting X86_FEATURE_TSC_KNOWN_FREQ.
>
> Signed-off-by: Alexey Makhalov <amakhalov@...are.com>
> ---
>  arch/x86/kernel/cpu/vmware.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
> index c6ede3b3d302..83164110ccc5 100644
> --- a/arch/x86/kernel/cpu/vmware.c
> +++ b/arch/x86/kernel/cpu/vmware.c
> @@ -378,6 +378,8 @@ static void __init vmware_set_capabilities(void)
>  {
>  	setup_force_cpu_cap(X86_FEATURE_CONSTANT_TSC);
>  	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> +	if (vmware_tsc_khz)
> +		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	if (vmware_hypercall_mode == CPUID_VMWARE_FEATURES_ECX_VMCALL)
>  		setup_force_cpu_cap(X86_FEATURE_VMCALL);
>  	else if (vmware_hypercall_mode == CPUID_VMWARE_FEATURES_ECX_VMMCALL)

The same trick is being used in Xen/Jailhouse/KVM code already and
Hyper-V overwrites x86_platform.calibrate_tsc/x86_platform.calibrate_cpu
hooks to basically achive the same goal. Should we maybe introduce a
flag in 'struct hypervisor_x86' or something like that to unify all
this?

Just a suggestion.

-- 
Vitaly