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Date: Tue, 15 Jun 2010 14:41:10 +0300
From: Avi Kivity <avi@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC: Marcelo Tosatti <mtosatti@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 5/6] KVM: MMU: prefetch ptes when intercepted guest #PF
On 06/15/2010 05:47 AM, Xiao Guangrong wrote:
> Support prefetch ptes when intercept guest #PF, avoid to #PF by later
> access
>
> If we meet any failure in the prefetch path, we will exit it and
> not try other ptes to avoid become heavy path
>
>
>
> +#define PTE_PREFETCH_NUM 16
> +
> #define PT_FIRST_AVAIL_BITS_SHIFT 9
> #define PT64_SECOND_AVAIL_BITS_SHIFT 52
>
> @@ -2041,6 +2043,39 @@ static void nonpaging_new_cr3(struct kvm_vcpu *vcpu)
> {
> }
>
> +static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
> +{
> + struct kvm_mmu_page *sp;
> + int index, i;
> +
> + sp = page_header(__pa(sptep));
> + WARN_ON(!sp->role.direct);
> + index = sptep - sp->spt;
> +
> + for (i = index + 1; i< min(PT64_ENT_PER_PAGE,
> + index + PTE_PREFETCH_NUM); i++) {
> + gfn_t gfn;
> + pfn_t pfn;
> + u64 *spte = sp->spt + i;
> +
> + if (*spte != shadow_trap_nonpresent_pte)
> + continue;
> +
> + gfn = sp->gfn + (i<< ((sp->role.level - 1) * PT64_LEVEL_BITS));
>
Can calculate outside the loop and use +=.
Can this in fact work for level != PT_PAGE_TABLE_LEVEL? We might start
at PT_PAGE_DIRECTORY_LEVEL but get 4k pages while iterating.
> +
> + pfn = gfn_to_pfn_atomic(vcpu->kvm, gfn);
> + if (is_error_pfn(pfn)) {
> + kvm_release_pfn_clean(pfn);
> + break;
> + }
> + if (pte_prefetch_topup_memory_cache(vcpu))
> + break;
> +
> + mmu_set_spte(vcpu, spte, ACC_ALL, ACC_ALL, 0, 0, 1, NULL,
> + sp->role.level, gfn, pfn, true, false);
> + }
> +}
>
Nice. Direct prefetch should usually succeed.
Can later augment to call get_users_pages_fast(..., PTE_PREFETCH_NUM,
...) to reduce gup overhead.
>
> +static void FNAME(pte_prefetch)(struct kvm_vcpu *vcpu, u64 *sptep)
> +{
> + struct kvm_mmu_page *sp;
> + pt_element_t *table = NULL;
> + int offset = 0, shift, index, i;
> +
> + sp = page_header(__pa(sptep));
> + index = sptep - sp->spt;
> +
> + if (PTTYPE == 32) {
> + shift = PAGE_SHIFT - (PT_LEVEL_BITS -
> + PT64_LEVEL_BITS) * sp->role.level;
> + offset = sp->role.quadrant<< shift;
> + }
> +
> + for (i = index + 1; i< min(PT64_ENT_PER_PAGE,
> + index + PTE_PREFETCH_NUM); i++) {
> + struct page *page;
> + pt_element_t gpte;
> + unsigned pte_access;
> + u64 *spte = sp->spt + i;
> + gfn_t gfn;
> + pfn_t pfn;
> + int dirty;
> +
> + if (*spte != shadow_trap_nonpresent_pte)
> + continue;
> +
> + pte_access = sp->role.access;
> + if (sp->role.direct) {
> + dirty = 1;
> + gfn = sp->gfn + (i<< ((sp->role.level - 1) *
> + PT64_LEVEL_BITS));
> + goto gfn_mapping;
> + }
>
Should just call direct_pte_prefetch.
> +
> + if (!table) {
> + page = gfn_to_page_atomic(vcpu->kvm, sp->gfn);
> + if (is_error_page(page)) {
> + kvm_release_page_clean(page);
> + break;
> + }
> + table = kmap_atomic(page, KM_USER0);
> + table = (pt_element_t *)((char *)table + offset);
> + }
>
Why not kvm_read_guest_atomic()? Can do it outside the loop.
> +
> + gpte = table[i];
> + if (!(gpte& PT_ACCESSED_MASK))
> + continue;
> +
> + if (!is_present_gpte(gpte)) {
> + if (!sp->unsync)
> + *spte = shadow_notrap_nonpresent_pte;
>
Need __set_spte().
> + continue;
> + }
> + dirty = is_dirty_gpte(gpte);
> + gfn = (gpte& PT64_BASE_ADDR_MASK)>> PAGE_SHIFT;
> + pte_access = pte_access& FNAME(gpte_access)(vcpu, gpte);
> +gfn_mapping:
> + pfn = gfn_to_pfn_atomic(vcpu->kvm, gfn);
> + if (is_error_pfn(pfn)) {
> + kvm_release_pfn_clean(pfn);
> + break;
> + }
> +
> + if (pte_prefetch_topup_memory_cache(vcpu))
> + break;
> + mmu_set_spte(vcpu, spte, sp->role.access, pte_access, 0, 0,
> + dirty, NULL, sp->role.level, gfn, pfn,
> + true, false);
> + }
> + if (table)
> + kunmap_atomic((char *)table - offset, KM_USER0);
> +}
>
I think lot of code can be shared with the pte prefetch in invlpg.
> +
> /*
> * Fetch a shadow pte for a specific level in the paging hierarchy.
> */
> @@ -322,6 +397,7 @@ static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
> is_dirty_gpte(gw->ptes[gw->level-1]),
> ptwrite, level,
> gw->gfn, pfn, false, true);
> + FNAME(pte_prefetch)(vcpu, sptep);
> break;
> }
>
>
--
error compiling committee.c: too many arguments to function
--
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