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Commit 86a5ba02 authored by Avi Kivity's avatar Avi Kivity Committed by Linus Torvalds
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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: default avatarAvi Kivity <avi@qumranet.com>
Acked-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 139bdb2d

drivers/kvm/kvm.h

+3 −0
Original line number Original line Diff line number Diff line
@@ -238,6 +238,9 @@ struct kvm_vcpu { 
	struct kvm_mmu_page page_header_buf[KVM_NUM_MMU_PAGES];

	struct kvm_mmu_page page_header_buf[KVM_NUM_MMU_PAGES];

	struct kvm_mmu mmu;

	struct kvm_mmu mmu;





	gfn_t last_pt_write_gfn;

	int   last_pt_write_count;



	struct kvm_guest_debug guest_debug;

	struct kvm_guest_debug guest_debug;





	char fx_buf[FX_BUF_SIZE];

	char fx_buf[FX_BUF_SIZE];

drivers/kvm/mmu.c

+15 −1
Original line number Original line Diff line number Diff line
@@ -969,8 +969,17 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes) 
	unsigned page_offset;

	unsigned page_offset;

	unsigned misaligned;

	unsigned misaligned;

	int level;

	int level;

	int flooded = 0;





	pgprintk("%s: gpa %llx bytes %d\n", __FUNCTION__, gpa, bytes);

	pgprintk("%s: gpa %llx bytes %d\n", __FUNCTION__, gpa, bytes);

	if (gfn == vcpu->last_pt_write_gfn) {

		++vcpu->last_pt_write_count;

		if (vcpu->last_pt_write_count >= 3)

			flooded = 1;

	} else {

		vcpu->last_pt_write_gfn = gfn;

		vcpu->last_pt_write_count = 1;

	}

	index = kvm_page_table_hashfn(gfn) % KVM_NUM_MMU_PAGES;

	index = kvm_page_table_hashfn(gfn) % KVM_NUM_MMU_PAGES;

	bucket = &vcpu->kvm->mmu_page_hash[index];

	bucket = &vcpu->kvm->mmu_page_hash[index];

	hlist_for_each_entry_safe(page, node, n, bucket, hash_link) {

	hlist_for_each_entry_safe(page, node, n, bucket, hash_link) {
@@ -978,11 +987,16 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes) 
			continue;

			continue;

		pte_size = page->role.glevels == PT32_ROOT_LEVEL ? 4 : 8;

		pte_size = page->role.glevels == PT32_ROOT_LEVEL ? 4 : 8;

		misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);

		misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);

		if (misaligned) {

		if (misaligned || flooded) {

			/*

			/*

			 * Misaligned accesses are too much trouble to fix

			 * Misaligned accesses are too much trouble to fix

			 * up; also, they usually indicate a page is not used

			 * up; also, they usually indicate a page is not used

			 * as a page table.

			 * as a page table.

			 *

			 * If we're seeing too many writes to a page,

			 * it may no longer be a page table, or we may be

			 * forking, in which case it is better to unmap the

			 * page.

			 */

			 */

			pgprintk("misaligned: gpa %llx bytes %d role %x\n",

			pgprintk("misaligned: gpa %llx bytes %d role %x\n",

				 gpa, bytes, page->role.word);

				 gpa, bytes, page->role.word);