Re: [PATCH v7 2/3] x86, apicv: add virtual interrupt delivery support

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 20, 2012 at 03:12:32PM +0200, Gleb Natapov wrote:
> On Thu, Dec 20, 2012 at 10:53:16AM -0200, Marcelo Tosatti wrote:
> > On Thu, Dec 20, 2012 at 08:42:06AM +0200, Gleb Natapov wrote:
> > > On Wed, Dec 19, 2012 at 10:59:36PM -0200, Marcelo Tosatti wrote:
> > > > On Mon, Dec 17, 2012 at 01:30:49PM +0800, Yang Zhang wrote:
> > > > > From: Yang Zhang <yang.z.zhang@Intel.com>
> > > > > 
> > > > > Virtual interrupt delivery avoids KVM to inject vAPIC interrupts
> > > > > manually, which is fully taken care of by the hardware. This needs
> > > > > some special awareness into existing interrupr injection path:
> > > > > 
> > > > > - for pending interrupt, instead of direct injection, we may need
> > > > >   update architecture specific indicators before resuming to guest.
> > > > > 
> > > > > - A pending interrupt, which is masked by ISR, should be also
> > > > >   considered in above update action, since hardware will decide
> > > > >   when to inject it at right time. Current has_interrupt and
> > > > >   get_interrupt only returns a valid vector from injection p.o.v.
> > > > > 
> > > > > Signed-off-by: Kevin Tian <kevin.tian@intel.com>
> > > > > Signed-off-by: Yang Zhang <yang.z.zhang@intel.com>
> > > > 
> > > > 
> > > > Resuming previous discussion:
> > > > 
> > > > > > How about to recaculate irr_pending according the VIRR on each
> > > > > > vmexit?
> > > > > > 
> > > > > No need really. Since HW can only clear VIRR the only situation that
> > > > > may
> > > > > happen is that irr_pending will be true but VIRR is empty and
> > > > > apic_find_highest_irr() will return correct result in this case.
> > > > 
> > > > Self-IPI does cause VIRR to be set, see "29.1.5 Self-IPI 
> > > > Virtualization".
> > > >
> > > True. But as I said later in that discussion once irr_pending is set
> > > to true it never becomes false, so the optimization is effectively
> > > disable. We can set it to true doing apic initialization to make it
> > > explicit.
> > 
> > Its just confusing, to have a variable which has different meanings
> > in different configurations. I would rather have it explicit that 
> > its not used rather than check every time the i read the code.
> > 
> > if (apic_vid() == 0 && !apic->irr_pending)
> > 	return -1;
> > 
> I'd prefer to avoid this additional if() especially as its sole purpose
> is documentation.  We can add comment instead. Note that irr_pending
> is just a hint anyway.  It can be true when no interrupt is pending in
> irr. We can even rename it to irr_pending_hint or something.

Works for me (documentation).

> > Not sure if you can skip it, its probably necessary to calculate it
> > before HW does so (say migration etc).
> kvm_apic_has_interrupt() is not called during migration and
> kvm_apic_post_state_restore() calls apic_update_ppr() explicitly.
> I am not sure it is needed though since migrated value should be already
> correct anyway.

Ok, best force isr_count to 1 if apic vintr enabled (and add a comment,
please).