On 05/23/2012 11:55 AM, Xiao Guangrong wrote:
> If the the present bit of page fault error code is set, it indicates
> the shadow page is populated on all levels, it means what we do is
> only modify the access bit which can be done out of mmu-lock
>
> Currently, in order to simplify the code, we only fix the page fault
> caused by write-protect on the fast path
>
> +
> static bool spte_has_volatile_bits(u64 spte)
> {
> + /*
> + * Always atomicly update spte if it can be updated
> + * out of mmu-lock.
> + */
> + if (spte_can_lockless_update(spte))
> + return true;
This is a really subtle point, but is it really needed?
Lockless spte updates should always set the dirty and accessed bits, so
we won't be overwriting any volatile bits there.
> +
> if (!shadow_accessed_mask)
> return false;
>
> @@ -498,13 +517,7 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
> return ret;
> }
>
> - new_spte |= old_spte & shadow_dirty_mask;
> -
> - mask = shadow_accessed_mask;
> - if (is_writable_pte(old_spte))
> - mask |= shadow_dirty_mask;
> -
> - if (!spte_has_volatile_bits(old_spte) || (new_spte & mask) == mask)
> + if (!spte_has_volatile_bits(old_spte))
> __update_clear_spte_fast(sptep, new_spte);
> else
> old_spte = __update_clear_spte_slow(sptep, new_spte);
It looks like the old code is bad.. why can we ignore volatile bits in
the old spte? Suppose pfn is changing?
> +
> +static bool
> +fast_pf_fix_indirect_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> + u64 *sptep, u64 spte, gfn_t gfn)
> +{
> + pfn_t pfn;
> + bool ret = false;
> +
> + /*
> + * For the indirect spte, it is hard to get a stable gfn since
> + * we just use a cmpxchg to avoid all the races which is not
> + * enough to avoid the ABA problem: the host can arbitrarily
> + * change spte and the mapping from gfn to pfn.
> + *
> + * What we do is call gfn_to_pfn_atomic to bind the gfn and the
> + * pfn because after the call:
> + * - we have held the refcount of pfn that means the pfn can not
> + * be freed and be reused for another gfn.
> + * - the pfn is writable that means it can not be shared by different
> + * gfn.
> + */
> + pfn = gfn_to_pfn_atomic(vcpu->kvm, gfn);
> +
> + /* The host page is swapped out or merged. */
> + if (mmu_invalid_pfn(pfn))
> + goto exit;
> +
> + ret = true;
> +
> + if (pfn != spte_to_pfn(spte))
> + goto exit;
> +
> + if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte)
> + mark_page_dirty(vcpu->kvm, gfn);
Isn't it better to kvm_release_pfn_dirty() here?
> +
> +exit:
> + kvm_release_pfn_clean(pfn);
> + return ret;
> +}
> +
> +> +
> +/*
> + * Return value:
> + * - true: let the vcpu to access on the same address again.
> + * - false: let the real page fault path to fix it.
> + */
> +static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn,
> + int level, u32 error_code)
> +{
> + struct kvm_shadow_walk_iterator iterator;
> + struct kvm_mmu_page *sp;
> + bool ret = false;
> + u64 spte = 0ull;
> +
> + if (!page_fault_can_be_fast(vcpu, gfn, error_code))
> + return false;
> +
No need to pass gfn here.
> + walk_shadow_page_lockless_begin(vcpu);
> + for_each_shadow_entry_lockless(vcpu, gva, iterator, spte)
> + if (!is_shadow_present_pte(spte) || iterator.level < level)
> + break;
> +
> + /*
> + * If the mapping has been changed, let the vcpu fault on the
> + * same address again.
> + */
> + if (!is_rmap_spte(spte)) {
> + ret = true;
> + goto exit;
> + }
> +
> + if (!is_last_spte(spte, level))
> + goto exit;
> +
> + /*
> + * Check if it is a spurious fault caused by TLB lazily flushed.
> + *
> + * Need not check the access of upper level table entries since
> + * they are always ACC_ALL.
> + */
> + if (is_writable_pte(spte)) {
> + ret = true;
> + goto exit;
> + }
> +
> + /*
> + * Currently, to simplify the code, only the spte write-protected
> + * by dirty-log can be fast fixed.
> + */
> + if (!spte_can_be_writable(spte))
> + goto exit;
> +
> + sp = page_header(__pa(iterator.sptep));
> +
> + if (sp->role.direct)
> + ret = fast_pf_fix_direct_spte(vcpu, sp, iterator.sptep, spte);
> + else
> + ret = fast_pf_fix_indirect_spte(vcpu, sp, iterator.sptep,
> + spte, gfn);
> +
> +exit:
> + walk_shadow_page_lockless_end(vcpu);
> +
> + return ret;
> +}
> +
--
error compiling committee.c: too many arguments to function
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