linux-kernel - Re: [PATCH v2 15/15] KVM: x86: Factor out kvm_use_master

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Date: Wed, 1 May 2024 10:55:46 -0700
From: "Chen, Zide" <zide.chen@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Paul Durrant
 <paul@....org>, Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Oliver Upton <oliver.upton@...ux.dev>, Marcelo Tosatti
 <mtosatti@...hat.com>, jalliste@...zon.co.uk, sveith@...zon.de,
 Dongli Zhang <dongli.zhang@...cle.com>
Subject: Re: [PATCH v2 15/15] KVM: x86: Factor out kvm_use_master_clock()



On 4/27/2024 4:05 AM, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Both kvm_track_tsc_matching() and pvclock_update_vm_gtod_copy() make a
> decision about whether the KVM clock should be in master clock mode.
> 
> They use *different* criteria for the decision though. This isn't really
> a problem; it only has the potential to cause unnecessary invocations of
> KVM_REQ_MASTERCLOCK_UPDATE if the masterclock was disabled due to TSC
> going backwards, or the guest using the old MSR. But it isn't pretty.
> 
> Factor the decision out to a single function. And document the historical
> reason why it's disabled for guests that use the old MSR_KVM_SYSTEM_TIME.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++----
>  1 file changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d6e4469f531a..680b39f17851 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2518,6 +2518,27 @@ static inline bool gtod_is_based_on_tsc(int mode)
>  }
>  #endif
>  
> +static bool kvm_use_master_clock(strut kvm *kvm)

typo: 'strut' -> 'struct'

> +{
> +	struct kvm_arch *ka = &kvm->arch;
> +
> +	/*
> +	 * The 'old kvmclock' check is a workaround (from 2015) for a
> +	 * SUSE 2.6.16 kernel that didn't boot if the system_time in
> +	 * its kvmclock was too far behind the current time. So the
> +	 * mode of just setting the reference point and allowing time
> +	 * to proceed linearly from there makes it fail to boot.
> +	 * Despite that being kind of the *point* of the way the clock
> +	 * is exposed to the guest. By coincidence, the offending
> +	 * kernels used the old MSR_KVM_SYSTEM_TIME, which was moved
> +	 * only because it resided in the wrong number range. So the
> +	 * workaround is activated for *all* guests using the old MSR.
> +	 */
> +	return ka->all_vcpus_matched_tsc &&
> +		!ka->backwards_tsc_observed &&
> +		!ka->boot_vcpu_runs_old_kvmclock;
> +}
> +
>  static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
>  {
>  #ifdef CONFIG_X86_64
> @@ -2550,7 +2571,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
>  	 * To use the masterclock, the host clocksource must be based on TSC
>  	 * and all vCPUs must have matching TSC frequencies.
>  	 */
> -	bool use_master_clock = ka->all_vcpus_matched_tsc &&
> +	bool use_master_clock = kvm_use_master_clock(kvm) &&

'kvm' should be `vcpu->kvm`

>  				gtod_is_based_on_tsc(gtod->clock.vclock_mode);