Subject: Re: [RFC PATCH 1/1] vhost: TX used buffer guest signal accumulation
From: Shirley Ma <mashirle@us.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
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Date: Wed, 03 Nov 2010 22:38:46 -0700
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On Wed, 2010-11-03 at 12:48 +0200, Michael S. Tsirkin wrote:
> I mean in practice, you see a benefit from this patch?

Yes, I tested it. It does benefit the performance.

> > My concern here is whether checking only in set up would be
> sufficient
> > for security?
> 
> It better be sufficient because the checks that put_user does
> are not effictive when run from the kernel thread, anyway.
> 
> > Would be there is a case guest could corrupt the ring
> > later? If not, that's OK.
> 
> You mean change the pointer after it's checked?
> If you see such a case, please holler.

I wonder about it, not a such case in mind.

> To clarify: the combination of __put_user and separate
> signalling is giving the same performance benefit as your
> patch?

Yes, it has similar performance, not I haven't finished all message
sizes comparison yet.

> I am mostly concerned with adding code that seems to help
> speed for reasons we don't completely understand, because
> then we might break the optimization easily without noticing.

I don't think the patch I submited would break up anything. It just
reduced the cost of per used buffer 3 put_user() calls and guest
signaling from one to one to many to one.

Thanks
Shirley

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