From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH v7 2/3] x86, apicv: add virtual interrupt delivery support
Date: Wed, 19 Dec 2012 22:59:36 -0200
Message-ID: <20121220005936.GA21033@amt.cnet>
References: <1355722250-7122-1-git-send-email-yang.z.zhang@intel.com>
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Cc: kvm@vger.kernel.org, gleb@redhat.com, haitao.shan@intel.com,
	Kevin Tian <kevin.tian@intel.com>
To: Yang Zhang <yang.z.zhang@intel.com>
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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".

Also, an example of problem with ISR caching optimization (which was written
with ISR/IRR management entirely in software):

isr_count variable is never incremented (because HW sets ISR):
kvm_cpu_get_interrupt does not call into kvm_get_apic_interrupt 
if virtual interrupt delivery enabled.

Therefore apic_find_highest_isr can return -1 even though VISR
is not zero.

In that case (VISR non-zero but apic_find_highest_isr() == -1), 
apic_update_ppr does not correctly set

PPR = max(ISRV, TPR)

Which can result in kvm_arch_vcpu_runnable returning 1 when it should not.

Please disable usage of isr_count if virtual interrupt delivery enabled.
Given that self-IPI writes to VIRR, also please disable usage of
highest_isr_cache and irr_pending.