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

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Message-ID: <84F586C5-EECE-4EE5-8988-64D8E0325D7A@vmware.com>
Date:   Mon, 15 Mar 2021 15:24:43 -0700
From:   Alexey Makhalov <amakhalov@...are.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     <linux-kernel@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        <mingo@...hat.com>, <tglx@...utronix.de>, <pv-drivers@...are.com>,
        <sdeep@...are.com>
Subject: Re: [PATCH] x86/vmware: avoid TSC recalibration

Hi Vitaly,

I believe, it is responsibility of each guest code to set X86_FEATURE_TSC_KNOWN_FREQ cap.
Regarding VMware guest, there is a case where vmware_tsc_khz is zero (not provided by hypervisor)
and TSC frequency should be calculated.

Sorry for late response.

Regards,
—Alexey

> On Jan 5, 2021, at 5:06 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> 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