[Xen-devel] Ping: [PATCH] x86: use VMLOAD for PV context switch

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.08.18 at 00:04, <brian.woods@xxxxxxx> wrote:
> On Tue, Jul 10, 2018 at 04:14:11AM -0600, Jan Beulich wrote:
>> Having noticed that VMLOAD alone is about as fast as a single of the
>> involved WRMSRs, I thought it might be a reasonable idea to also use it
>> for PV. Measurements, however, have shown that an actual improvement can
>> be achieved only with an early prefetch of the VMCB (thanks to Andrew
>> for suggesting to try this), which I have to admit I can't really
>> explain. This way on my Fam15 box context switch takes over 100 clocks
>> less on average (the measured values are heavily varying in all cases,
>> though).
>> 
>> This is intentionally not using a new hvm_funcs hook: For one, this is
>> all about PV, and something similar can hardly be done for VMX.
>> Furthermore the indirect to direct call patching that is meant to be
>> applied to most hvm_funcs hooks would be ugly to make work with
>> functions having more than 6 parameters.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I have confirmed it with a senior hardware engineer and using vmload in
> this fashion is safe and recommended for performance.  As far as using
> vmload with PV.
> 
> Acked-by: Brian Woods <brian.woods@xxxxxxx>

Thanks. Andrew?

Jan



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