Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU

[Ben Gardon]

On Wed, Aug 25, 2021 at 3:49 PM Lai Jiangshan
<jiangshanlai+lkml@xxxxxxxxx> wrote:
>
> On Thu, Aug 19, 2021 at 7:57 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > Make a final call to direct_pte_prefetch_many() if there are "trailing"
> > SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN.  The
> > call to direct_pte_prefetch_many() in the loop only handles the case
> > where there are !PRESENT SPTEs preceding a PRESENT SPTE.
> >
> > E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all
> > SPTEs for the following GFNs are !PRESENT, the loop will terminate with
> > "start = sptep+1" and not prefetch any SPTEs.
> >
> > Prefetching trailing SPTEs as intended can drastically reduce the number
> > of guest page faults, e.g. accessing the first byte of every 4kb page in
> > a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory,
> > the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M.
> >
> > Note, this only affects memory that is backed by 4kb pages as KVM doesn't
> > prefetch when installing hugepages.  Shadow paging prefetching is not
> > affected as it does not batch the prefetches due to the need to process
> > the corresponding guest PTE.  The TDP MMU is not affected because it
> > doesn't have prefetching, yet...
> >
> > Fixes: 957ed9effd80 ("KVM: MMU: prefetch ptes when intercepted guest #PF")
> > Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxx>
> > Cc: Ben Gardon <bgardon@xxxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>

> > ---
> >
> > Cc'd Ben as this highlights a potential gap with the TDP MMU, which lacks
> > prefetching of any sort.  For large VMs, which are likely backed by
> > hugepages anyways, this is a non-issue as the benefits of holding mmu_lock
> > for read likely masks the cost of taking more VM-Exits.  But VMs with a
> > small number of vCPUs won't benefit as much from parallel page faults,
> > e.g. there's no benefit at all if there's a single vCPU.

Yeah, that probably does represent a reduction in performance for very
small VMs. Besides keeping read critical sections small, there's no
reason not to do prefetching with the TDP MMU, it just needs to be
implemented.

> >
> >  arch/x86/kvm/mmu/mmu.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index a272ccbddfa1..daf7df35f788 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2818,11 +2818,13 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
> >                         if (!start)
> >                                 continue;
> >                         if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0)
> > -                               break;
> > +                               return;
> >                         start = NULL;
> >                 } else if (!start)
> >                         start = spte;
> >         }
> > +       if (start)
> > +               direct_pte_prefetch_many(vcpu, sp, start, spte);

It might be worth explaining some of what you laid out in the commit
description here. This function's implementation is not the easiest to
read.

> >  }
>
>
> Reviewed-by: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
>
> >
> >  static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
> > --
> > 2.33.0.rc1.237.g0d66db33f3-goog
> >