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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.



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