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[PATCH] KVM: MMU: Page table write flood protection

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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>
This commit is contained in:
Avi Kivity 2007-01-05 16:36:50 -08:00 committed by Linus Torvalds
parent 139bdb2d9e
commit 86a5ba025d
2 changed files with 18 additions and 1 deletions
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3
drivers/kvm/kvm.h
View File

@ -238,6 +238,9 @@ struct kvm_vcpu {
struct kvm_mmu_page page_header_buf[KVM_NUM_MMU_PAGES];
struct kvm_mmu mmu;
gfn_t last_pt_write_gfn;
int last_pt_write_count;
struct kvm_guest_debug guest_debug;
char fx_buf[FX_BUF_SIZE];

16
drivers/kvm/mmu.c
View File

@ -969,8 +969,17 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
unsigned page_offset;
unsigned misaligned;
int level;
int flooded = 0;
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;
bucket = &vcpu->kvm->mmu_page_hash[index];
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;
pte_size = page->role.glevels == PT32_ROOT_LEVEL ? 4 : 8;
misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);
if (misaligned) {
if (misaligned || flooded) {
/*
* Misaligned accesses are too much trouble to fix
* up; also, they usually indicate a page is not used
* 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",
gpa, bytes, page->role.word);