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

[Brian Woods <brian.woods@xxxxxxx>]

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>

-- 
Brian Woods

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