Re: [Xen-devel] [PATCH for-4.5 v11 0/9] Mem_event and mem_access for ARM

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Mon, Sep 29, 2014 at 5:07 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

On Mon, 2014-09-29 at 16:21 +0200, Tamas K Lengyel wrote:

> The patch you reference in the previous round was newly added has been
> refactored in this round to avoid adding overhead. If it's your
> feeling that there might be some other similar cases and you want to
> delay so you have more time to look at it just be sure, that's
> perfectly understandable, but IMHO in this version there is no
> indication that we are adding any unreasonable overhead.

As I say I think we need to step back and take our time over this. I
think 4.6 is the right target for this stuff, otherwise we are rushing
and risking slipping in something which has an unexpected impact.

Ack.

 

> I don't see how those benchmarks would be meaningful for this series.
> During normal operations, the only overhead for the domain would be in
> the trap handlers checking the boolean flag if mem_access is in use in
> case a permission fault happened in the second stage translation..
> which I have never observed happening during my tests. So those
> benchmarks don't really exercise any paths that mem_access touches.

It touches the p2m update code which is a hot path. Also it previously
touched the copy to/from guest paths which is super hot, if you aren't
doing that anymore then great, if you are then there is still a
potential for regressions.

But in any case the benchmarks will serve to highlight *unexpected*
regressions to serve as confirmation of what you expect.

OK.

 

For example I think they would would have pretty clearly shown poor
performance due to the copy to/from user changes in your previous
iteration.

Ian.

You are probably right, but aren't the copy to/from helpers only used if the guest is issuing hypercalls? I mean, what would the hypervisor copy in/out of the guest during kernbench running in the guest? That doesn't really sound right to me.

Tamas

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