On 09/10/2018 10:03 AM, 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 <jbeul...@suse.com>
> Acked-by: Brian Woods <brian.wo...@amd.com>
> ---
> v2: Re-base.
> ---
> Besides the mentioned oddity with measured performance, I've also
> noticed a significant difference (of at least 150 clocks) between
> measuring immediately around the calls to svm_load_segs() and measuring
> immediately inside the function.
>
>
> +#ifdef CONFIG_PV
> +bool svm_load_segs(unsigned int ldt_ents, unsigned long ldt_base,
> + unsigned int fs_sel, unsigned long fs_base,
> + unsigned int gs_sel, unsigned long gs_base,
> + unsigned long gs_shadow)
> +{
> + unsigned int cpu = smp_processor_id();
> + struct vmcb_struct *vmcb = per_cpu(host_vmcb_va, cpu);
> +
> + if ( unlikely(!vmcb) )
> + return false;
> +
> + if ( !ldt_base )
> + {
> + asm volatile ( "prefetch %0" :: "m" (vmcb->ldtr) );
> + return true;
Could you explain why this is true? We haven't loaded FS/GS here.
I also couldn't find discussion about prefetch --- why is prefetching
ldtr expected to help?
-boris
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