linux-kernel - Re: [PATCH] KVM: x86: always exit on EOIs for interrupts listed in the IOAPIC redir table

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Date:	Wed, 06 Aug 2014 16:08:38 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: always exit on EOIs for interrupts listed in
 the IOAPIC redir table

Il 06/08/2014 16:03, Zhang, Yang Z ha scritto:
> Paolo Bonzini wrote on 2014-07-31:
>> Probably, the guest is masking the interrupt in the redirection table in
>> the interrupt routine, i.e. while the interrupt is set in a LAPIC's ISR.
>> The simplest fix is to ignore the masking state, we would rather have
>> an unnecessary exit rather than a missed IRQ ACK and anyway IOAPIC
>> interrupts are not as performance-sensitive as for example MSIs.
> 
> I feel this fixing may hurt performance in some cases. If the mask
> bit is set, this means the vector in this entry may be used by other
> devices(like a assigned device). But here you set it in eoi exit bitmap
> and this will cause vmexit on each EOI which should not happen.

Note that this *was* reported on an assigned device.

IOAPIC should not be a performance-sensitive path.  High-performance
assigned devices should be using MSIs.

Paolo
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