Re: [PATCH V3] perf & kvm: Enhance perf to collect KVM guest os statistics from host side

Avi Kivity
2010-04-17T18:12:41Z

On 04/15/2010 05:08 PM, Sheng Yang wrote:
> On Thursday 15 April 2010 18:44:15 Avi Kivity wrote:
>
>> On 04/15/2010 01:40 PM, Joerg Roedel wrote:
>>
>>>> That means an NMI that happens outside guest code (for example, in the
>>>> mmu, or during the exit itself) would be counted as if in guest code.
>>>>
>>> Hmm, true. The same is true for an NMI that happens between VMSAVE and
>>> STGI but that window is smaller. Anyway, I think we don't need the
>>> busy-wait loop. The NMI should be executed at a well defined point and
>>> we set the cpu_var back to NULL after that point.
>>>
>> The point is not well defined. Considering there are already at least
>> two implementations svm, I don't want to rely on implementation details.
>>
> After more investigating, I realized that I had interpreted the SDM wrong.
> Sorry.
>
> There is *no* risk with the original method of calling "int $2".
>
> According to the SDM 24.1:
>
>
>> The following bullets detail when architectural state is and is not updated
>>
> in response to VM exits:
> [...]
>
>> - An NMI causes subsequent NMIs to be blocked, but only after the VM exit
>>
> completes.
>
> So the truth is, after NMI directly caused VMExit, the following NMIs would be
> blocked, until encountered next "iret". So execute "int $2" is safe in
> vmx_complete_interrupts(), no risk in causing nested NMI. And it would unblock
> the following NMIs as well due to "iret" it executed.
>
> So there is unnecessary to make change to avoid "potential nested NMI".
>

Let's look at the surrounding text...

>
> The following bullets detail when architectural state is and is not
> updated in response
> to VM exits:
> • If an event causes a VM exit directly, it does not update
> architectural state as it
> would have if it had it not caused the VM exit:
> — A debug exception does not update DR6, DR7.GD, or IA32_DEBUGCTL.LBR.
> (Information about the nature of the debug exception is saved
> in the exit
> qualification field.)
> — A page fault does not update CR2. (The linear address causing
> the page fault
> is saved in the exit-qualification field.)
> — An NMI causes subsequent NMIs to be blocked, but only after the
> VM exit
> completes.
> — An external interrupt does not acknowledge the interrupt
> controller and the
> interrupt remains pending, unless the “acknowledge interrupt
> on exit”
> VM-exit control is 1. In such a case, the interrupt controller
> is acknowledged
> and the interrupt is no longer pending.

Everywhere it says state is _not_ updated, so I think what is meant is
that NMIs are blocked, but only _until_ the VM exit completes.

I think you were right the first time around. Can you check with your
architecture team?

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.