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

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Message-ID: <566EE708.6010404@oracle.com>
Date:	Mon, 14 Dec 2015 10:58:00 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	3.14+@...r.us.oracle.com, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, david.vrabel@...rix.com, jbeulich@...e.com,
	xen-devel@...ts.xenproject.org, #@...r.us.oracle.com
Subject: Re: [Xen-devel] [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

On 12/14/2015 10:35 AM, Roger Pau Monné wrote:
> El 14/12/15 a les 16.27, Konrad Rzeszutek Wilk ha escrit:
>> 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.

OK, I then mis-read the hypervisor code, I didn't realize that 
vcpumask_to_pcpumask() takes into account vcpu_dirty_cpumask.


>>
>>> 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.
>>
>> Perhaps if we have PV aware TLB flush it could do this differently?
> Why don't HVM/PVH just uses the HVMOP_flush_tlbs hypercall?

It doesn't take any parameters so it will invalidate TLBs for all VCPUs, 
which is more than is being asked for. Especially in the case of 
MMUEXT_INVLPG_MULTI.

(That's in addition to the fact that it currently doesn't work for PVH 
as it has a test for is_hvm_domain() instead of has_hvm_container_domain()).

-boris
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