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    SubjectRe: [RFC PATCH 1/1] vhost: TX used buffer guest signal accumulation
    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.


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