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KVM: MMU: Improve iteration through sptes from rmap 

Iteration using rmap_next(), the actual body is pte_list_next(), is
inefficient: every time we call it we start from checking whether rmap
holds a single spte or points to a descriptor which links more sptes.

In the case of shadow paging, this quadratic total iteration cost is a
problem.  Even for two dimensional paging, with EPT/NPT on, in which we
almost always have a single mapping, the extra checks at the end of the
iteration should be eliminated.

This patch fixes this by introducing rmap_iterator which keeps the
iteration context for the next search.  Furthermore the implementation
of rmap_next() is splitted into two functions, rmap_get_first() and
rmap_get_next(), to avoid repeatedly checking whether the rmap being
iterated on has only one spte.

Although there seemed to be only a slight change for EPT/NPT, the actual
improvement was significant: we observed that GET_DIRTY_LOG for 1GB
dirty memory became 15% faster than before.  This is probably because
the new code is easy to make branch predictions.

Note: we just remove pte_list_next() because we can think of parent_ptes
as a reverse mapping.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Takuya Yoshikawa 2012-03-21 23:50:34 +09:00 committed by Avi Kivity
parent 220f773a00
commit 1e3f42f03c
2 changed files with 124 additions and 82 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

196
arch/x86/kvm/mmu.c
View File

@ -842,32 +842,6 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
return count;
}
static u64 *pte_list_next(unsigned long *pte_list, u64 *spte)
{
struct pte_list_desc *desc;
u64 *prev_spte;
int i;
if (!*pte_list)
return NULL;
else if (!(*pte_list & 1)) {
if (!spte)
return (u64 *)*pte_list;
return NULL;
}
desc = (struct pte_list_desc *)(*pte_list & ~1ul);
prev_spte = NULL;
while (desc) {
for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i) {
if (prev_spte == spte)
return desc->sptes[i];
prev_spte = desc->sptes[i];
}
desc = desc->more;
}
return NULL;
}
static void
pte_list_desc_remove_entry(unsigned long *pte_list, struct pte_list_desc *desc,
int i, struct pte_list_desc *prev_desc)
@ -988,11 +962,6 @@ static int rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
return pte_list_add(vcpu, spte, rmapp);
}
static u64 *rmap_next(unsigned long *rmapp, u64 *spte)
{
return pte_list_next(rmapp, spte);
}
static void rmap_remove(struct kvm *kvm, u64 *spte)
{
struct kvm_mmu_page *sp;
@ -1005,6 +974,67 @@ static void rmap_remove(struct kvm *kvm, u64 *spte)
pte_list_remove(spte, rmapp);
}
/*
* Used by the following functions to iterate through the sptes linked by a
* rmap. All fields are private and not assumed to be used outside.
*/
struct rmap_iterator {
/* private fields */
struct pte_list_desc *desc; /* holds the sptep if not NULL */
int pos; /* index of the sptep */
};
/*
* Iteration must be started by this function. This should also be used after
* removing/dropping sptes from the rmap link because in such cases the
* information in the itererator may not be valid.
*
* Returns sptep if found, NULL otherwise.
*/
static u64 *rmap_get_first(unsigned long rmap, struct rmap_iterator *iter)
{
if (!rmap)
return NULL;
if (!(rmap & 1)) {
iter->desc = NULL;
return (u64 *)rmap;
}
iter->desc = (struct pte_list_desc *)(rmap & ~1ul);
iter->pos = 0;
return iter->desc->sptes[iter->pos];
}
/*
* Must be used with a valid iterator: e.g. after rmap_get_first().
*
* Returns sptep if found, NULL otherwise.
*/
static u64 *rmap_get_next(struct rmap_iterator *iter)
{
if (iter->desc) {
if (iter->pos < PTE_LIST_EXT - 1) {
u64 *sptep;
++iter->pos;
sptep = iter->desc->sptes[iter->pos];
if (sptep)
return sptep;
}
iter->desc = iter->desc->more;
if (iter->desc) {
iter->pos = 0;
/* desc->sptes[0] cannot be NULL */
return iter->desc->sptes[iter->pos];
}
}
return NULL;
}
static void drop_spte(struct kvm *kvm, u64 *sptep)
{
if (mmu_spte_clear_track_bits(sptep))
@ -1013,23 +1043,27 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
static int __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
{
u64 *spte = NULL;
u64 *sptep;
struct rmap_iterator iter;
int write_protected = 0;
while ((spte = rmap_next(rmapp, spte))) {
BUG_ON(!(*spte & PT_PRESENT_MASK));
rmap_printk("rmap_write_protect: spte %p %llx\n", spte, *spte);
for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
BUG_ON(!(*sptep & PT_PRESENT_MASK));
rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
if (!is_writable_pte(*spte))
if (!is_writable_pte(*sptep)) {
sptep = rmap_get_next(&iter);
continue;
}
if (level == PT_PAGE_TABLE_LEVEL) {
mmu_spte_update(spte, *spte & ~PT_WRITABLE_MASK);
mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
sptep = rmap_get_next(&iter);
} else {
BUG_ON(!is_large_pte(*spte));
drop_spte(kvm, spte);
BUG_ON(!is_large_pte(*sptep));
drop_spte(kvm, sptep);
--kvm->stat.lpages;
spte = NULL;
sptep = rmap_get_first(*rmapp, &iter);
}
write_protected = 1;
@ -1084,48 +1118,57 @@ static int rmap_write_protect(struct kvm *kvm, u64 gfn)
static int kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp,
unsigned long data)
{
u64 *spte;
u64 *sptep;
struct rmap_iterator iter;
int need_tlb_flush = 0;
while ((spte = rmap_next(rmapp, NULL))) {
BUG_ON(!(*spte & PT_PRESENT_MASK));
rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", spte, *spte);
drop_spte(kvm, spte);
while ((sptep = rmap_get_first(*rmapp, &iter))) {
BUG_ON(!(*sptep & PT_PRESENT_MASK));
rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", sptep, *sptep);
drop_spte(kvm, sptep);
need_tlb_flush = 1;
}
return need_tlb_flush;
}
static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
unsigned long data)
{
u64 *sptep;
struct rmap_iterator iter;
int need_flush = 0;
u64 *spte, new_spte;
u64 new_spte;
pte_t *ptep = (pte_t *)data;
pfn_t new_pfn;
WARN_ON(pte_huge(*ptep));
new_pfn = pte_pfn(*ptep);
spte = rmap_next(rmapp, NULL);
while (spte) {
BUG_ON(!is_shadow_present_pte(*spte));
rmap_printk("kvm_set_pte_rmapp: spte %p %llx\n", spte, *spte);
for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
BUG_ON(!is_shadow_present_pte(*sptep));
rmap_printk("kvm_set_pte_rmapp: spte %p %llx\n", sptep, *sptep);
need_flush = 1;
if (pte_write(*ptep)) {
drop_spte(kvm, spte);
spte = rmap_next(rmapp, NULL);
drop_spte(kvm, sptep);
sptep = rmap_get_first(*rmapp, &iter);
} else {
new_spte = *spte &~ (PT64_BASE_ADDR_MASK);
new_spte = *sptep & ~PT64_BASE_ADDR_MASK;
new_spte |= (u64)new_pfn << PAGE_SHIFT;
new_spte &= ~PT_WRITABLE_MASK;
new_spte &= ~SPTE_HOST_WRITEABLE;
new_spte &= ~shadow_accessed_mask;
mmu_spte_clear_track_bits(spte);
mmu_spte_set(spte, new_spte);
spte = rmap_next(rmapp, spte);
mmu_spte_clear_track_bits(sptep);
mmu_spte_set(sptep, new_spte);
sptep = rmap_get_next(&iter);
}
}
if (need_flush)
kvm_flush_remote_tlbs(kvm);
@ -1184,7 +1227,8 @@ void kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte)
static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
unsigned long data)
{
u64 *spte;
u64 *sptep;
struct rmap_iterator iter;
int young = 0;
/*
@ -1197,25 +1241,24 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
if (!shadow_accessed_mask)
return kvm_unmap_rmapp(kvm, rmapp, data);
spte = rmap_next(rmapp, NULL);
while (spte) {
int _young;
u64 _spte = *spte;
BUG_ON(!(_spte & PT_PRESENT_MASK));
_young = _spte & PT_ACCESSED_MASK;
if (_young) {
for (sptep = rmap_get_first(*rmapp, &iter); sptep;
sptep = rmap_get_next(&iter)) {
BUG_ON(!(*sptep & PT_PRESENT_MASK));
if (*sptep & PT_ACCESSED_MASK) {
young = 1;
clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)sptep);
}
spte = rmap_next(rmapp, spte);
}
return young;
}
static int kvm_test_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
unsigned long data)
{
u64 *spte;
u64 *sptep;
struct rmap_iterator iter;
int young = 0;
/*
@ -1226,16 +1269,14 @@ static int kvm_test_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
if (!shadow_accessed_mask)
goto out;
spte = rmap_next(rmapp, NULL);
while (spte) {
u64 _spte = *spte;
BUG_ON(!(_spte & PT_PRESENT_MASK));
young = _spte & PT_ACCESSED_MASK;
if (young) {
for (sptep = rmap_get_first(*rmapp, &iter); sptep;
sptep = rmap_get_next(&iter)) {
BUG_ON(!(*sptep & PT_PRESENT_MASK));
if (*sptep & PT_ACCESSED_MASK) {
young = 1;
break;
}
spte = rmap_next(rmapp, spte);
}
out:
return young;
@ -1887,10 +1928,11 @@ static void kvm_mmu_put_page(struct kvm_mmu_page *sp, u64 *parent_pte)
static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp)
{
u64 *parent_pte;
u64 *sptep;
struct rmap_iterator iter;
while ((parent_pte = pte_list_next(&sp->parent_ptes, NULL)))
drop_parent_pte(sp, parent_pte);
while ((sptep = rmap_get_first(sp->parent_ptes, &iter)))
drop_parent_pte(sp, sptep);
}
static int mmu_zap_unsync_children(struct kvm *kvm,

10
arch/x86/kvm/mmu_audit.c
View File

@ -192,7 +192,8 @@ static void audit_write_protection(struct kvm *kvm, struct kvm_mmu_page *sp)
{
struct kvm_memory_slot *slot;
unsigned long *rmapp;
u64 *spte;
u64 *sptep;
struct rmap_iterator iter;
if (sp->role.direct || sp->unsync || sp->role.invalid)
return;
@ -200,13 +201,12 @@ static void audit_write_protection(struct kvm *kvm, struct kvm_mmu_page *sp)
slot = gfn_to_memslot(kvm, sp->gfn);
rmapp = &slot->rmap[sp->gfn - slot->base_gfn];
spte = rmap_next(rmapp, NULL);
while (spte) {
if (is_writable_pte(*spte))
for (sptep = rmap_get_first(*rmapp, &iter); sptep;
sptep = rmap_get_next(&iter)) {
if (is_writable_pte(*sptep))
audit_printk(kvm, "shadow page has writable "
"mappings: gfn %llx role %x\n",
sp->gfn, sp->role.word);
spte = rmap_next(rmapp, spte);
}
}