linux-kernel - Re: [PATCH v2] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE

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Message-ID: <45bd5778-6217-427e-8ac2-f7b997470476@xen.org>
Date:   Wed, 1 Nov 2023 11:06:25 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        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>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM x86/xen: add an override for
 PVCLOCK_TSC_STABLE_BIT

On 31/10/2023 23:06, David Woodhouse wrote:
[snip]
>>                  mutex_lock(&kvm->lock);
>>                  if (!kvm->created_vcpus) {
>>                          kvm->arch.force_tsc_unstable = true;
>>                          r = 0;
>>                  }
>>                  mutex_unlock(&kvm->lock);
>>
>> So that it would be blatantly obvious that there's no race with checking a per-VM
>> flag without any lock/RCU protections.
> 
> Makes sense. Although TBH if the VMM wants to flip this bit on and off
> at runtime while the guest clocks are being updated, it deserves what
> it gets. It's not a problem for KVM.
> 

The first version of the patch that used an attribute requested a clock 
update when the attribute was set. I dropped that in this version but I 
think it'd best to re-instate it.

   Paul