linux-kernel - Re: [PATCH v2 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK

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Date: Wed, 10 Apr 2024 13:43:25 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, Paul Durrant
 <xadimgnik@...il.com>, Jack Allister <jalliste@...zon.com>
Cc: bp@...en8.de, corbet@....net, dave.hansen@...ux.intel.com, hpa@...or.com,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, mingo@...hat.com, pbonzini@...hat.com,
 seanjc@...gle.com, tglx@...utronix.de, x86@...nel.org,
 Dongli Zhang <dongli.zhang@...cle.com>
Subject: Re: [PATCH v2 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate
 KVM clock migration

On 10/04/2024 13:09, David Woodhouse wrote:
> On 10 April 2024 11:29:13 BST, Paul Durrant <xadimgnik@...il.com> wrote:
>> On 10/04/2024 10:52, Jack Allister wrote:
>>> +	 * It's possible that this vCPU doesn't have a HVCLOCK configured
>>> +	 * but the other vCPUs may. If this is the case calculate based
>>> +	 * upon the time gathered in the seqcount but do not update the
>>> +	 * vCPU specific PVTI. If we have one, then use that.
>>
>> Given this is a per-vCPU ioctl, why not fail in the case the vCPU doesn't have HVCLOCK configured? Or is your intention that a GET/SET should always work if TSC is stable?
> 
> It definitely needs to work for SET even when the vCPU hasn't been run yet (and doesn't have a hvclock in vcpu->arch.hv_clock).

So would it make sense to set up hvclock earlier?