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

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Date:	Wed, 2 Sep 2015 22:18:34 -0700
From:	"Nakajima, Jun" <jun.nakajima@...el.com>
To:	Steve Rutherford <srutherford@...gle.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"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>,
	"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 Wed, Sep 2, 2015 at 3:38 PM, Steve Rutherford <srutherford@...gle.com> wrote:
> On Thu, Aug 13, 2015 at 09:31:48AM +0200, Paolo Bonzini wrote:
> Pinging this thread.
>
> Should I put together a patch to make split irqchip work properly with the old TMR behavior?

Yes, please.

Intel® 64 and IA-32 Architectures Software Developer’s Manual:

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.
Software should ensure that each such data structure is modified only
when no logical processor with a current
VMCS that references it is in VMX non-root operation. Doing otherwise
may lead to unpredictable behavior
(including behaviors identified in Section 24.11.1)

29.6 POSTED-INTERRUPT PROCESSING
...
Use of the posted-interrupt descriptor differs from that of other data
structures that are referenced by pointers in
a VMCS. There is a general requirement that software ensure that each
such data structure is modified only when
no logical processor with a current VMCS that references it is in VMX
non-root operation. That requirement does
not apply to the posted-interrupt descriptor. There is a requirement,
however, that such modifications be done
using locked read-modify-write instructions.

>
>>
>>
>> On 13/08/2015 08:35, Zhang, Yang Z wrote:
>> >> You may be right. It is safe if no future hardware plans to use
>> >> it. Let me check with our hardware team to see whether it will be
>> >> used or not in future.
>> >
>> > After checking with Jun, there is no guarantee that the guest running
>> > on another CPU will operate properly if hypervisor modify the vTMR
>> > from another CPU. So the hypervisor should not to do it.
>>
>> I guess I can cause a vmexit on level-triggered interrupts, it's not a
>> big deal, but no weasel words, please.
>>
>> What's going to break, and where is it documented?
>>
>> Paolo
>> --
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-- 
Jun
Intel Open Source Technology Center
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