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

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Date:	Tue, 15 Dec 2015 08:24:40 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Boris Ostrovsky" <boris.ostrovsky@...cle.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 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.

Jan

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