Re: [PATCH 14/15] KVM: MMU: mmio page fault support

Avi Kivity
2011-06-12T08:38:10Z

On 06/10/2011 06:47 AM, Xiao Guangrong wrote:
> > Also, shadow walking is not significantly faster than guest page table walking. And if we miss, we have to walk the guest page tables in any case.
> >
>
> Um. i think walking guest page table is slower, it needs to walk memslots for many times
> and it triggers page fault if the host page is swapped.

Well, if the page is swapped, we can't store anything in the spte.

And if we only store the mmio/ram condition in the spte (via the two
types of page faults) we don't need to walk the spte. We know
immediately if we need to search the slots or not.

> And it is hardly missed, since for tdp, it infrequency zaps shadow pages, for soft mmu,
> the mmio spte is always unsync, and in guest, the mmio region is always mapped by kernel,
> so it is infrequency to be update and lazily flushed.

We still get frequent mmio misses.

> >> +
> >> +static bool quickly_check_mmio_pf(struct kvm_vcpu *vcpu, u64 addr, bool direct)
> >> +{
> >> + if (direct&& vcpu_match_mmio_gpa(vcpu, addr))
> >> + return true;
> >> +
> >> + if (vcpu_match_mmio_gva(vcpu, addr))
> >> + return true;
> >> +
> >> + return false;
> >> +}
> >
> > There is also the case of nesting - it's not direct and it's not a gva.
> >
>
> If it is direct, we only need to compare the pga, and direct=0, we only need to
> compare gva, i'll fix the code to make it clear.

But for nested npt, we get the ngpa, not a gva.

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error compiling committee.c: too many arguments to function