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

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Message-ID: <55BB2A62.5090302@redhat.com>
Date:	Fri, 31 Jul 2015 09:57:22 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Steve Rutherford <srutherford@...gle.com>,
	"Zhang, Yang Z" <yang.z.zhang@...el.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



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.

Paolo
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