Re: [PATCH 7/7] KVM: X86: Also prefetch the last range in __direct_pte_prefetch().

[Sean Christopherson]

On Tue, Aug 24, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> __direct_pte_prefetch() skips prefetching the last range.
> 
> The last range are often the whole range after the faulted spte when
> guest is touching huge-page-mapped(in guest view) memory forwardly
> which means prefetching them can reduce pagefault.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e5932af6f11c..ac260e01e9d8 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2847,8 +2847,9 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
>  	i = (sptep - sp->spt) & ~(PTE_PREFETCH_NUM - 1);
>  	spte = sp->spt + i;
>  
> -	for (i = 0; i < PTE_PREFETCH_NUM; i++, spte++) {
> -		if (is_shadow_present_pte(*spte) || spte == sptep) {
> +	for (i = 0; i <= PTE_PREFETCH_NUM; i++, spte++) {
> +		if (i == PTE_PREFETCH_NUM ||
> +		    is_shadow_present_pte(*spte) || spte == sptep) {

Heh, I posted a fix just a few days ago.  I prefer having a separate call after
the loop.  The "<= PTE_PREFETCH_NUM" is subtle, and a check at the ends avoids
a CMP+Jcc in the loop, though I highly doubt that actually affects performance.

https://lkml.kernel.org/r/20210818235615.2047588-1-seanjc@xxxxxxxxxx

>  			if (!start)
>  				continue;
>  			if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0)
> -- 
> 2.19.1.6.gb485710b
>