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

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Date:	Tue, 4 Aug 2015 00:46:10 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"srutherford@...el.com" <srutherford@...el.com>,
	"Gudimetla, Giridhar Kumar" <giridhar.kumar.gudimetla@...el.com>
Subject: RE: [PATCH 1/2] KVM: x86: set TMR when the interrupt is accepted

Paolo Bonzini wrote on 2015-08-03:
> 
> 
> On 03/08/2015 12:23, Zhang, Yang Z wrote:
>>> In any case, the TMR behavior introduced by the APICv patches is
>>> completely different from the hardware behavior, so it has to be fixed.
>> 
>> But any real problem with it?
> 
> It is a problem for split irqchip, where the EOI exit bitmap can be
> inferred from the IOAPIC routes but the TMR cannot.  The hardware
> behavior on the other hand can be implemented purely within the LAPIC.

So updating the TMR within LAPIC is the only solution to handle it?

> 
>>>  The alternative is to inject level-triggered interrupts
>>> synchronously, without using posted interrupts.
>>> 
>>> I'll write some testcases to understand the functioning of TMR in
>>> the virtual-APIC page, but the manual seems clear to me.
>> 
>> Currently, no existing hardware will use TMR and will not cause any
>> problem.(That's the reason why we leave it in Xen).But we don't know
>> whether future hardware will use it or not(SDM always keeps changing
>> :)).
> 
> But that would be covered by a different execution control (for
> backwards compatibility).  We'll get there when such a feature is introduced.

Yes, we can leave it in future. But one concern is that it may hard to handle it at that time if someone also develops feature which rely on it (like current patch to split irqchip). 

> 
>> And per 24.11.4's description, the perfect solution is don't modify
>> it. btw, IIRC, only TMR doesn't follow the rule. All other VMCS
>> accesses are issued in right VMCS context.
> 
> Yes, that's correct.  It's just the TMR.
> 
> Paolo


Best regards,
Yang


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