Re: [PATCH] tools/libxc: Avoid hypercalls for cacheflush on x86

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/03/14 01:14, Ian Campbell wrote:
> On Mon, 2014-03-03 at 16:22 +0000, Ian Jackson wrote:
>> Andrew Cooper writes ("[PATCH] tools/libxc: Avoid hypercalls for cacheflush on x86"):
>>> XEN_DOMCTL_cacheflush hypercalls are -ENOSYS on x86.  As domain
>>> creation/migration is already very hypercall-heavy, avoid making extra
>>> redundant hypercalls
>> Surely this should be done by moving the relevant code to an
>> arm-specific file, not by #ifdeffery.
> I think it is very debatable that sort-circuiting a single ENOSYS
> hypercall in the tools is even worth it at all. "very hypercall heavy"
> doesn't cut it as a rationale IMHO. Unless an actual demonstrable
> improvement is seen, perhaps over a larger series removing a large
> number of so called redundant calls, it's just an additional source of
> confusion (due to gating at multiple levels) as far as I'm concerned.
>
> Ian.
>

For booting a single guest, noone will notice.  Booting 10 or more at a
time however is very different.

XenServer, along with all other virtualisation platforms live in a world
where "Net time to boot $N guests" (where N is measured in hundreds) is
an important quantity, and needless overhead from things like this cause
dom0 needless load that it would rather do without.

I should say that I have not done any performance metrics on this - as
noted in the patch I stumbled upon it with valgrind.

~Andrew