linux-kernel - RE: [PATCH 1/2] KVM: x86: set TMR when the interrupt is accepted

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Date:	Mon, 3 Aug 2015 02:44:06 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Steve Rutherford <srutherford@...gle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"srutherford@...el.com" <srutherford@...el.com>
Subject: RE: [PATCH 1/2] KVM: x86: set TMR when the interrupt is accepted

Paolo Bonzini wrote on 2015-07-31:
> 
> 
> On 31/07/2015 04:49, Steve Rutherford wrote:
>> Oh... Yeah. That's a damn good point, given that the interrupt can be
>> injected from another thread while one is in that guest vcpu.
>> 
>> Easiest time to update the TMR should be on guest entry through
>> vcpu_scan_ioapic, as before.
>> 
>> The best way to go is probably to ditch the new per vcpu EOI exit
>> bitmap, and just update/use the TMR. There's no reason to duplicate
>> that data in the representation of the apic (I suspect that the
>> duplication was inspired by my mistaken notion of the TMR). The
>> IOAPIC exit check can use the TMR instead.
>> 
>> Based upon my reading of the SDM, the only reason that the eoi exit
>> bitmaps are not the exact same as the TMR is that it is possible to
>> have virtual-interrupt delivery enabled /without/ an apic access page
>> (Note: V-ID => EOI exit bitmap enabled).
>> 
>> Yang, do you happen to know if that is the case?
>> 
>> [Note: Just looked back at the old code for updating the EOI exit
>> bitmaps, which for some reason was configured to trigger EOI exits
>> for all IOAPIC interrupts, not just level-triggered IOAPIC
>> interrupts. Which is weird, and I believe totally unecessary.]
> 
> The RTC interrupt needs to trigger an EOI exit with the in-kernel
> IOAPIC, in order to detect coalesced interrupts.  This is necessary to
> avoid severe time drift in Windows guest.

Correct! EOI exit bitmap is not absolutely same with TMR. Some interrupts (especially timer interrupt) which is edge trigger but need a notification when it got injected into guest. 

> 
> Paolo


Best regards,
Yang


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