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

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Message-ID: <7c7238a9c8b0dc6bc865407ba804a651cdfdb044.camel@infradead.org>
Date:   Wed, 22 Nov 2023 10:39:04 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....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 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?
> 
> The point in your previous patch which split out a shiny new
> set_shinfo_evtchn_pending() function was that you could then *call*
> that function to ensure that the corresponding index bit was set on the
> destination host after migration, if the bit in the shinfo is.
> 
> So we'd do that from kvm_xen_setup_evtchn(), kvm_xen_eventfd_assign(),
> and when setting KVM_XEN_VCPU_ATTR_TYPE_TIMER.
> 
>  if (bit_is_set_in_shinfo)
>    set_shinfo_evtchn_pending()

I mean set_vcpu_info_evtchn_pending() of course. And we probably want
to extend the xen_shinfo_test to test it, by setting the bit in the
shinfo to mark the event as pending, and then doing each of

 • Set up timer (a bit like in TEST_TIMER_RESTORE at line 817).
 • Add incoming eventfd with KVM_SET_GSI_ROUTING (cf. line 563)
 • Add IPI with KVM_XEN_ATTR_TYPE_EVTCHN (cf. line 597)

Each of those should set the index bit in the vcpu_info immediately if
the evtchn port is already set (and unmasked) in the shinfo.


(Ignore this part if you're cleverer than me or have had more coffee…)

It took me a moment to get my head around the different setups we have
for event channels, but that's because the standard KVM_SET_GSI_ROUTING
one is for *incoming* events, just as we would for MSIs, and we use the
standard way of attaching an eventfd to an incoming GSI/MSI/evtchn.

The KVM_XEN_ATTR_TYPE_EVTCHN one is for *outbound* events where the
guest does an EVTCHNOP_send. That can *raise* events on an eventfd, or
it can be an IPI or loopback interdomain port, which is the case we
need to test.

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