linux-kernel - Re: [PATCH v3 2/4] KVM: x86: Use vector-hashing to deliver lowest-priority interrupts

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Date:	Fri, 22 Jan 2016 14:49:12 +0100
From:	"rkrcmar@...hat.com" <rkrcmar@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	"Wu, Feng" <feng.wu@...el.com>,
	"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 2/4] KVM: x86: Use vector-hashing to deliver
 lowest-priority interrupts

2016-01-22 12:00+0800, Yang Zhang:
> On 2016/1/22 1:21, rkrcmar@...hat.com wrote:
>>(I think there isn't a practical difference between *r=-1 and *r=0.)
> 
> Currently, if *r == -1, the remote_irr may get set. But it seems wrong. I

Yeah ...

> need to have a double check to see whether it is a bug in current code.

Looking forward to the patch!

Thanks.

>>'ret = true' is the better one.  We know that the interrupt is not
>>deliverable [1], so there's no point in trying to deliver with the slow
>>path.  We behave similarly when the interrupt targets a single disabled
>>APIC.
>>
>>---
>>1: Well ... it's possible that slowpath would deliver it thanks to
>>    different handling of disabled APICs, but it's undefined behavior,
> 
> why it is undefined behavior? Besides, why we will keep two different
> handling logic for the fast path and slow path? It looks weird.

It does look very weird ... the slow path would require refactoring,
though, so we save effort without a considerable drawback.
(I would love if it behaved identically, but I don't want to force it on
 someone and likely won't do it myself ...)

I consider it undefined because SMD says that an OS musn't configure
this behavior and doesn't say what should happen if the OS does => we
could do anything.  (Killing the guest would be great for debugging OS
issues, but ours behavior is fairly conservative.)