linux-kernel - Re: [PATCH v3 3/4] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Date:	Fri, 22 Jan 2016 15:07:32 +0100
From:	Radim Krcmár <rkrcmar@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>
Cc:	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts

2016-01-22 05:12+0000, Wu, Feng:
>> From: Radim Krčmář [mailto:rkrcmar@...hat.com]
>> 2016-01-20 09:42+0800, Feng Wu:
>>> -	if (kvm_intr_is_single_vcpu_fast(kvm, irq, dest_vcpu))
>>> +	if (kvm_intr_can_posting_fast(kvm, irq, dest_vcpu))
>>>  		return true;
>> 
>> There is one pitfall:  xAPIC flat logical broadcast returns false,
> 
> Do you mean kvm_intr_can_posting_fast() returns false for
> xAPIC flat logical lowest-priority broadcast?

I did.

> After carefully read the code for several times, I still cannot
> find the reason, could you please give more hints?

You are right, there isn't a problem in the code.

> BTW, I noticed there is  a "if(irq->dest_id == 0xFF) goto out;" in
> this function, but it is for the physical dest mode. I am not
> sure you mean this.

I didn't check if my assumptions were wrong.  I'm sorry.

>> but lowest priority is defined for it (practically isn't a broadcast) and
>> the rest of this function doesn't check for lowest priority, so the
>> interrupt won't be posted.
>> 
>> We could modify our _fast functions to cover 0xff in flat logical, but
>> ignoring this case isn't bad either ... it can happen only with 8 VCPU
>> guests. 
> 
> Could you please elaborate a bit more why only for the 8 VCPU guests?

xAPIC flat logical doesn't forbid lowest priority broadcasts, but lowest
priority delivery still needs to have all destinations enabled, which
can only happen with 8 VCPUs.