linux-kernel - Re: [PATCH v2 08/12] KVM: xen: automatically use the vcpu_info embedded in shared

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Date:   Mon, 18 Sep 2023 13:15:00 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v2 08/12] KVM: xen: automatically use the vcpu_info
 embedded in shared_info

On 18/09/2023 12:49, David Woodhouse wrote:
> On Mon, 2023-09-18 at 11:21 +0000, Paul Durrant wrote:
>> -                                              This is because KVM may
>> -  not be aware of the Xen CPU id which is used as the index into the
>> -  vcpu_info[] array, so may know the correct default location.
> 
> Hm, that *was* true at the time of writing, but we did end up teaching
> KVM about the Xen vcpu_id so that we can handle timers. (We required
> userspace to provide the APIC ID for all the other event channel stuff,
> but timer hypercalls come straight from the guest).
> 
> But I think the *only* thing we use vcpu->arch.xen.vcpu_id for right
> now is acceleration of the timer hypercalls that are restricted to the
> vCPU that they're called from, e.g.:
> 
> 	case VCPUOP_set_singleshot_timer:
> 		if (vcpu->arch.xen.vcpu_id != vcpu_id) {
> 			*r = -EINVAL;
> 
> So it's never mattered much before now if the Xen vcpu_id changes at
> runtime.
> 
> Maybe you now want to invalidate the vcpu_info pfncache for a vCPU if
> its Xen vcpu_id changes? Or just *forbid* such a change after the
> shinfo page has been set up? What locking prevents the xen.vcpu_id
> changing in the middle of your new get_vcpu_info_cache() function?

That's a good point; the vcpu lock itself won't be enough. I think 
forbidding a change of id while a shared_info page is in place is 
probably the most sensible semantic.

   Paul