linux-kernel - Re: [PATCH v8 15/15] KVM: xen: allow vcpu

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Date:   Wed, 22 Nov 2023 10:55:30 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 15/15] KVM: xen: allow vcpu_info content to be 'safely'
 copied

On 22/11/2023 10:39, David Woodhouse wrote:
> On Tue, 2023-11-21 at 22:53 +0000, David Woodhouse wrote:
>> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
>>> From: Paul Durrant <pdurrant@...zon.com>
>>>
>>> If the guest sets an explicit vcpu_info GPA then, for any of the first 32
>>> vCPUs, the content of the default vcpu_info in the shared_info page must be
>>> copied into the new location. Because this copy may race with event
>>> delivery (which updates the 'evtchn_pending_sel' field in vcpu_info) there
>>> needs to be a way to defer that until the copy is complete.
>>> Happily there is already a shadow of 'evtchn_pending_sel' in kvm_vcpu_xen
>>> that is used in atomic context if the vcpu_info PFN cache has been
>>> invalidated so that the update of vcpu_info can be deferred until the
>>> cache can be refreshed (on vCPU thread's the way back into guest context).
>>>
>>> Also use this shadow if the vcpu_info cache has been *deactivated*, so that
>>> the VMM can safely copy the vcpu_info content and then re-activate the
>>> cache with the new GPA. To do this, stop considering an inactive vcpu_info
>>> cache as a hard error in kvm_xen_set_evtchn_fast().
>>>
>>> Signed-off-by: Paul Durrant <pdurrant@...zon.com>
>>> Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
>>
>> Wait, didn't we realise that this leaves the bits set in the shadow
>> evtchn_pending_sel that get lost on migration?
>>

Indeed we did not, but that's not something that *this* patch, or even 
this series, is dealing with.  We also know that setting the 'width' of 
shared_info has some issues, but again, can we keep that for other 
patches? The series is at v9 and has already suffered a fair amount of 
scope-creep.

   Paul