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

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Message-ID: <55BF21E1.5030100@redhat.com>
Date:	Mon, 3 Aug 2015 10:10:09 +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>
Subject: Re: [PATCH 1/2] KVM: x86: set TMR when the interrupt is accepted

On 03/08/2015 04:37, Zhang, Yang Z wrote:
>> > Only virtualized APIC register reads use the virtual TMR registers (SDM
>> > 29.4.2 or 29.5), but these just read data from the corresponding field
>> > in the virtual APIC page.
> 
> 24.11.4 Software Access to Related Structures
> In addition to data in the VMCS region itself, VMX non-root operation
> can be controlled by data structures that are referenced by pointers
> in a VMCS (for example, the I/O bitmaps). While the pointers to these
> data structures are parts of the VMCS, the data structures themselves
> are not. They are not accessible using VMREAD and VMWRITE but by
> ordinary memory writes.

I don't think the TMR fields of the virtual-APIC page are among the data
structures that _controls_ VMX non-root operations, because:

* it is not part of the virtualized APIC state is listed in 29.1.1

* read accesses from the APIC-access page simply return data from the
corresponding page offset on the virtual-APIC page using the memory
access type stored in IA32_VMX_BASIC_MSR.  I think this explicitly says
that the effects of 24.11.1 (especially non-deterministic behavior after
a write) do not apply here.

In any case, the TMR behavior introduced by the APICv patches is
completely different from the hardware behavior, so it has to be fixed.
 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.

Paolo
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