Re: [Xen-devel] [PATCH v2 3/4] x86/nvmx: split updating RVI from SVI in nvmx_update_apicv

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: Thursday, March 26, 2020 5:20 PM
> 
> On Thu, Mar 26, 2020 at 03:13:56AM +0000, Tian, Kevin wrote:
> > > From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > > Sent: Wednesday, March 25, 2020 6:19 PM
> > >
> > > Updating SVI is required when an interrupt has been injected using the
> > > Ack on exit VMEXIT feature, so that the in service interrupt in the
> > > GUEST_INTR_STATUS matches the vector that is signaled in
> > > VM_EXIT_INTR_INFO.
> > >
> > > Updating RVI however is not tied to the Ack on exit feature, as it
> > > signals the next vector to be injected, and hence should always be
> > > updated to the next pending vector, regardless of whether Ack on exit
> > > is enabled.
> > >
> > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > ---
> > >  xen/arch/x86/hvm/vmx/vvmx.c | 22 +++++++++++++++++-----
> > >  1 file changed, 17 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/xen/arch/x86/hvm/vmx/vvmx.c
> b/xen/arch/x86/hvm/vmx/vvmx.c
> > > index 1753005c91..8431c912a1 100644
> > > --- a/xen/arch/x86/hvm/vmx/vvmx.c
> > > +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> > > @@ -1384,26 +1384,38 @@ static void nvmx_update_apicv(struct vcpu
> *v)
> > >      struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
> > >      unsigned long reason = get_vvmcs(v, VM_EXIT_REASON);
> > >      unsigned long intr_info = get_vvmcs(v, VM_EXIT_INTR_INFO);
> > > +    int rvi;
> > >
> > >      if ( reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
> > >           nvmx->intr.source == hvm_intsrc_lapic &&
> > >           (intr_info & INTR_INFO_VALID_MASK) )
> > >      {
> > >          uint16_t status;
> > > -        uint32_t rvi, ppr;
> > > -        uint32_t vector = intr_info & 0xff;
> > > +        uint32_t ppr;
> > > +        unsigned int vector = intr_info & INTR_INFO_VECTOR_MASK;
> > >          struct vlapic *vlapic = vcpu_vlapic(v);
> > >
> > > +        /*
> > > +         * Update SVI to record the currently in service interrupt that's
> > > +         * signaled in EXIT_INTR_INFO.
> > > +         */
> > >          vlapic_ack_pending_irq(v, vector, 1);
> > >
> > >          ppr = vlapic_set_ppr(vlapic);
> > >          WARN_ON((ppr & 0xf0) != (vector & 0xf0));
> > >
> > >          status = vector << VMX_GUEST_INTR_STATUS_SVI_OFFSET;
> > > -        rvi = vlapic_has_pending_irq(v);
> > > -        if ( rvi != -1 )
> > > -            status |= rvi & VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
> > > +        __vmwrite(GUEST_INTR_STATUS, status);
> > > +    }
> > > +
> > > +    rvi = vlapic_has_pending_irq(v);
> > > +    if ( rvi != -1 )
> > > +    {
> > > +        unsigned long status;
> > >
> > > +        __vmread(GUEST_INTR_STATUS, &status);
> > > +        status &= ~VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
> > > +        status |= rvi & VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
> > >          __vmwrite(GUEST_INTR_STATUS, status);
> > >      }
> >
> > I have two minor comments. First, original code requires only one
> > __vmwrite but now needs two writes and one read for Ack-on-exit
> > is set. Can we optimize to sustain the original behavior?
> 
> I think I can manage to do a single write by using a global status
> variable initialized to 0 and only do the vmwrite if the value is != 0
> by the end of the function.
> 
> > Second,
> > although I didn't come up a case where always updating RVI (w/
> > pending interrupt) may bring some bad effect, it is anyway safer
> > to put the whole logic within the check of vmexit reason and intr
> > source.
> 
> I also considered this and decided there was no harm in always
> updating RVI if there's a pending interrupt, it could even prevent a
> future VMEXIT to update RVI AFAICT?
> 
> If you prefer I can return early from the function if reason !=
> EXIT_REASON_EXTERNAL_INTERRUPT.
> 

I agree with your but given the trickiness of nested intr handling
I prefer to a more conservative but safer approach. So yes, return
early sounds good here.

Thanks
Kevin