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Re: [Xen-devel] [PATCH 2/3] x86/nvmx: clarify and fix usage of nvmx_update_apicv



On Tue, Mar 24, 2020 at 06:03:28AM +0000, Tian, Kevin wrote:
> > From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > Sent: Saturday, March 21, 2020 3:08 AM
> > 
> > The current usage of nvmx_update_apicv is not clear: it is deeply
> > intertwined with the Ack interrupt on exit VMEXIT control.
> > 
> > The code in nvmx_update_apicv should update the SVI (in service interrupt)
> > field of the guest interrupt status only when the Ack interrupt on
> > exit is set, as it is used to record that the interrupt being
> > serviced is signaled in a vmcs field, and hence hasn't been injected
> > as on native. It's important to record the current in service
> > interrupt on the guest interrupt status field, or else further
> > interrupts won't respect the priority of the in service one.
> > 
> > While clarifying the usage make sure that the SVI is only updated when
> > Ack on exit is set and the nested vmcs interrupt info field is valid. Or
> > else a guest not using the Ack on exit feature would loose interrupts as
> > they would be signaled as being in service on the guest interrupt
> > status field but won't actually be recorded on the interrupt info vmcs
> > field, neither injected in any other way.
> 
> It is insufficient. You also need to update RVI to enable virtual injection
> when Ack on exit is cleared.

But RVI should be updated in vmx_intr_assist in that case, since
nvmx_intr_intercept shouldn't intercept the interrupt, as it should be
handled normally.

> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/hvm/vmx/vvmx.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
> > index 1b8461ba30..180d01e385 100644
> > --- a/xen/arch/x86/hvm/vmx/vvmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> > @@ -1383,7 +1383,7 @@ static void (struct vcpu *v)
> >  {
> >      struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
> >      unsigned long reason = get_vvmcs(v, VM_EXIT_REASON);
> > -    uint32_t intr_info = nvmx->intr.intr_info;
> > +    unsigned long intr_info = get_vvmcs(v, VM_EXIT_INTR_INFO);
> 
> well, above reminds me an interesting question. Combining last and this
> patch, we'd see essentially that it goes back to the state before f96e1469
> (at least when Ack on exit is true).

Well, patch 1/3 is a revert of f96e1469, so just reverting f96e1469
gets us to that state.

This patch is an attempt to clarify that nvmx_update_apicv is closely
related to the Ack on exit feature, as it modifies SVI in order to
signal the vector currently being serviced by the EXIT_INTR_INFO vmcs
field. This was not obvious to me, as at first sight I assumed
nvmx_update_apicv was actually injecting that vector into the guest.

> iirc, that commit was introduced to enable
> nested x2apic with apicv, and your very first version even just removed
> the whole nvmx_update_apicv. Then now with the new reverted logic,
> are you still suffering x2apic problem? If not, does it imply the real fix
> is actually coming from patch 3/3 for eoi bitmap update?

Yes, patch 3/3 is the one that fixed the issue. Note however that
strangely enough removing the call to nvmx_update_apicv (as my first
attempt to solve the issue) did fix it on one of my boxes. It depends
a lot on the available vmx features AFAICT.

> > 
> >      if ( reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
> >           nvmx->intr.source == hvm_intsrc_lapic &&
> > @@ -1399,6 +1399,15 @@ static void nvmx_update_apicv(struct vcpu *v)
> >          ppr = vlapic_set_ppr(vlapic);
> >          WARN_ON((ppr & 0xf0) != (vector & 0xf0));
> > 
> > +        /*
> > +         * SVI must be updated when the interrupt has been signaled using 
> > the
> > +         * Ack on exit feature, or else the currently in-service interrupt
> > +         * won't be respected.
> > +         *
> > +         * Note that this is specific to the fact that when doing a VMEXIT 
> > an
> > +         * interrupt might get delivered using the interrupt info vmcs 
> > field
> > +         * instead of being injected normally.
> > +         */
> 
> I'm not sure mentioning SVI specifically here is necessary, since all steps
> here are required - updating iir, isr, rvi, svi, ppr, etc. It is just an 
> emulation
> of updating virtual APIC state as if a virtual interrupt delivery happens.

Hm, it was hard for me to figure out that SVI is modified here in
order to signal that the Ack on exit vector is currently in service,
as opposed to being injected using the virtual interrupt delivery
mechanism.

I just wanted to clarify that the purpose of this function is not to
inject the vector in intr_info, but rather to signal that such vector
has already been injected using a different mechanism.

I'm happy to reword this, but IMO it should be clear that the purpose
of the function is not so much to inject an interrupt, but to update
the virtual interrupt delivery field in order to signal that an
interrupt has been injected using a different mechanism.

Thanks, Roger.



 


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