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Re: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID

To: Wei Wang2 <wei.wang2@xxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID
From: Keir Fraser <keir@xxxxxxx>
Date: Wed, 12 Jan 2011 12:48:49 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID
User-agent: Microsoft-Entourage/
It begs the question whether it's worth complicating code for an
optimisation with no measurable benefit, doesn't it?

 -- Keir

On 12/01/2011 12:41, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:

> Hi Tim,
> Flush by ASID provides more flexible control of tlb flushing. The most
> advantage is to allow hypervisor to flush tagged tlb selectively. Using this
> feature, HV is able to flush tlb entries associated with a guest VM directly
> instead of allocating a new asid . The whole tlb flush will also be reduced
> by reducing asid allocation.
> So far, we did not measure drastic performance improvement in testing with
> kernbench and X11perf. Actually, we found out that, reducing tlb flushes
> accompanying with vmrun does not improve performance very much.
> we sent out a patch to optimize hvm_flush_guest_tlbs last week, which reduces
> over 90% tlb flushes for vmrun, and we even cannot see signification speedup
> with it. Maybe, the latency of vmrun is too big so that the overhead of tlb
> flush is negligible?
> Thanks,
> Wei
> On Wednesday 12 January 2011 11:17:00 Tim Deegan wrote:
>> At 17:55 +0000 on 11 Jan (1294768552), Wei Wang2 wrote:
>>> Future AMD SVM supports a new feature called flush by ASID. The idea is
>>> to allow CPU to flush TLBs associated with the ASID assigned to guest VM.
>>> So hypervisor doesn't have to reassign a new ASID in order to flush
>>> guest's VCPU.  Please review it.
>> What advantage does the new system have?  Intuitively it seems like it
>> might be a tiny bit fairer and a tiny bit faster (by explicitly flushing
>> instead of relying on LRO) but I'm not convinced that it will be visible
>> in macro-benchmarks.  Have you measured it?
>> Cheers,
>> Tim.
>>> Thanks,
>>> Wei
>>> Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
>>> Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
>>> --
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