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

On Mon, Sep 29, 2014 at 5:21 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
On Mon, 2014-09-29 at 17:17 +0200, Tamas K Lengyel wrote:

>         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.

Guests (and dom0 issue hypercalls as part of the PV ring protocols
(kicking evtchns, mapping and unmapping grant references, etc) and for a
variety of other things while they are running workloads. For almost all
of those operations some sort of argument struct normally needs to be
copied in/out (often both) of guest memory.


Ah, OK, that makes sense.

I'll resubmit the refactoring patches from this series that I think could still go into 4.5 shortly.

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