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

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Date:	Mon, 3 Aug 2015 12:55:32 +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>,
	"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



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.

> >  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.

> 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
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