linux-kernel - Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb

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Message-ID: <567033CF.105@oracle.com>
Date:	Tue, 15 Dec 2015 10:37:51 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	#@...r.us.oracle.com, 3.14+@...r.us.oracle.com,
	david.vrabel@...rix.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

On 12/15/2015 10:24 AM, Jan Beulich wrote:
>>>> On 15.12.15 at 16:14, <boris.ostrovsky@...cle.com> wrote:
>> On 12/15/2015 10:03 AM, Jan Beulich wrote:
>>>>>> On 15.12.15 at 15:36, <boris.ostrovsky@...cle.com> wrote:
>>>> On 12/14/2015 10:27 AM, Konrad Rzeszutek Wilk wrote:
>>>>> On Sat, Dec 12, 2015 at 07:25:55PM -0500, Boris Ostrovsky wrote:
>>>>>> Using MMUEXT_TLB_FLUSH_MULTI doesn't buy us much since the hypervisor
>>>>>> will likely perform same IPIs as would have the guest.
>>>>>>
>>>>> But if the VCPU is asleep, doing it via the hypervisor will save us waking
>>>>> up the guest VCPU, sending an IPI - just to do an TLB flush
>>>>> of that CPU. Which is pointless as the CPU hadn't been running the
>>>>> guest in the first place.
>>>>>
>>>>>> More importantly, using MMUEXT_INVLPG_MULTI may not to invalidate the
>>>>>> guest's address on remote CPU (when, for example, VCPU from another
>>>>>> guest
>>>>>> is running there).
>>>>> Right, so the hypervisor won't even send an IPI there.
>>>>>
>>>>> But if you do it via the normal guest IPI mechanism (which are opaque
>>>>> to the hypervisor) you and up scheduling the guest VCPU to do
>>>>> send an hypervisor callback. And the callback will go the IPI routine
>>>>> which will do an TLB flush. Not necessary.
>>>>>
>>>>> This is all in case of oversubscription of course. In the case where
>>>>> we are fine on vCPU resources it does not matter.
>>>> So then should we keep these two operations (MMUEXT_INVLPG_MULTI and
>>>> MMUEXT_TLB_FLUSH_MULT) available to HVM/PVH guests? If the guest's VCPU
>>>> is not running then TLBs must have been flushed.
>>> While I followed the discussion, it didn't become clear to me what
>>> uses these are for HVM guests considering the separate address
>>> spaces.
>> To avoid unnecessary IPIs to VCPUs that are not currently scheduled (my
>> mistake was that I didn't realize that IPIs to those pCPUs will be
>> filtered out by the hypervisor).
>>
>>> As long as they're useless if called, I'd still favor making
>>> them inaccessible.
>> VCPUs that are scheduled will receive the required flush requests.
> I don't follow - an INVLPG done by the hypervisor won't do any
> flushing for a HVM guest.

I thought that this would be done with VPID of intended VCPU still 
loaded and so INVLPG would flush guest's address?

-boris

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