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[PATCH] KVM: MMU: Page table write flood protection
In fork() (or when we protect a page that is no longer a page table), we can experience floods of writes to a page, which have to be emulated. This is expensive. So, if we detect such a flood, zap the page so subsequent writes can proceed natively. Signed-off-by: Avi Kivity <avi@qumranet.com> Acked-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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2 changed files with 18 additions and 1 deletions
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@ -238,6 +238,9 @@ struct kvm_vcpu {
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struct kvm_mmu_page page_header_buf[KVM_NUM_MMU_PAGES];
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struct kvm_mmu mmu;
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gfn_t last_pt_write_gfn;
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int last_pt_write_count;
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struct kvm_guest_debug guest_debug;
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char fx_buf[FX_BUF_SIZE];
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@ -969,8 +969,17 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
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unsigned page_offset;
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unsigned misaligned;
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int level;
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int flooded = 0;
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pgprintk("%s: gpa %llx bytes %d\n", __FUNCTION__, gpa, bytes);
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if (gfn == vcpu->last_pt_write_gfn) {
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++vcpu->last_pt_write_count;
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if (vcpu->last_pt_write_count >= 3)
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flooded = 1;
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} else {
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vcpu->last_pt_write_gfn = gfn;
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vcpu->last_pt_write_count = 1;
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}
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index = kvm_page_table_hashfn(gfn) % KVM_NUM_MMU_PAGES;
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bucket = &vcpu->kvm->mmu_page_hash[index];
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hlist_for_each_entry_safe(page, node, n, bucket, hash_link) {
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@ -978,11 +987,16 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
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continue;
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pte_size = page->role.glevels == PT32_ROOT_LEVEL ? 4 : 8;
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misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);
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if (misaligned) {
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if (misaligned || flooded) {
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/*
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* Misaligned accesses are too much trouble to fix
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* up; also, they usually indicate a page is not used
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* as a page table.
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*
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* If we're seeing too many writes to a page,
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* it may no longer be a page table, or we may be
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* forking, in which case it is better to unmap the
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* page.
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*/
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pgprintk("misaligned: gpa %llx bytes %d role %x\n",
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gpa, bytes, page->role.word);
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