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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH RFC] xen/gnttab: Avoid TLB flush if the grant mapped page was not accessed
Implement an x86 specific optimisation to avoid a TLB flush if the grant
mapped page was not accessed by a CPU whilst it was mapped.
The optimisation works by checking the accessed page bit of the pte to
determine if the page has been accessed by a cpu. The x86 specification details
that the processor atomically sets the accessed bit before caching a PTE into
a TLB.
The grant mapped pte is now setup without the accessed or dirty bit's set and
so we can determine if any CPU has accessed that page via the grant mapping
pte.
I believe Xen drops all attempts by guest to clear the accessed or dirty bit's
on the pte because Xen is the pte owner. This should prevent a malicous guest
from delibrately circumventing the TLB flush.
Blkback benefits from this optimistation because it does not access the data
itself from the CPU.
Netback can benefit from the optimistation depending on how much of the header
is put into the first fragment because the first fragment is grant copied on
recent netback implementations.
Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
diff -r 3e0ee081c616 -r 38f320029371 xen/arch/arm/mm.c
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1216,7 +1216,7 @@ int create_grant_host_mapping(unsigned l
}
int replace_grant_host_mapping(unsigned long addr, unsigned long mfn,
- unsigned long new_addr, unsigned int flags)
+ unsigned long new_addr, unsigned int flags, int *page_accessed)
{
unsigned long gfn = (unsigned long)(addr >> PAGE_SHIFT);
struct domain *d = current->domain;
@@ -1225,6 +1225,7 @@ int replace_grant_host_mapping(unsigned
return GNTST_general_error;
guest_physmap_remove_page(d, gfn, mfn, 0);
+ *page_accessed = 1;
return GNTST_okay;
}
diff -r 3e0ee081c616 -r 38f320029371 xen/arch/x86/mm.c
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3901,7 +3901,8 @@ static int create_grant_va_mapping(
}
static int replace_grant_va_mapping(
- unsigned long addr, unsigned long frame, l1_pgentry_t nl1e, struct vcpu *v)
+ unsigned long addr, unsigned long frame, l1_pgentry_t nl1e, struct vcpu *v,
+ int *page_accessed)
{
l1_pgentry_t *pl1e, ol1e;
unsigned long gl1mfn;
@@ -3947,12 +3948,13 @@ static int replace_grant_va_mapping(
}
/* Delete pagetable entry. */
- if ( unlikely(!UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, v, 0)) )
+ if ( unlikely(!UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, v, 1)) )
{
MEM_LOG("Cannot delete PTE entry at %p", (unsigned long *)pl1e);
rc = GNTST_general_error;
goto unlock_and_out;
}
+ *page_accessed = !!(l1e_get_flags(*pl1e) & _PAGE_ACCESSED);
unlock_and_out:
page_unlock(l1pg);
@@ -3963,9 +3965,9 @@ static int replace_grant_va_mapping(
}
static int destroy_grant_va_mapping(
- unsigned long addr, unsigned long frame, struct vcpu *v)
+ unsigned long addr, unsigned long frame, struct vcpu *v, int
*page_accessed)
{
- return replace_grant_va_mapping(addr, frame, l1e_empty(), v);
+ return replace_grant_va_mapping(addr, frame, l1e_empty(), v,
page_accessed);
}
static int create_grant_p2m_mapping(uint64_t addr, unsigned long frame,
@@ -4000,7 +4002,7 @@ int create_grant_host_mapping(uint64_t a
return create_grant_p2m_mapping(addr, frame, flags, cache_flags);
grant_pte_flags =
- _PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_DIRTY | _PAGE_GNTTAB;
+ _PAGE_PRESENT | _PAGE_GNTTAB;
if ( cpu_has_nx )
grant_pte_flags |= _PAGE_NX_BIT;
@@ -4048,7 +4050,8 @@ static int replace_grant_p2m_mapping(
}
int replace_grant_host_mapping(
- uint64_t addr, unsigned long frame, uint64_t new_addr, unsigned int flags)
+ uint64_t addr, unsigned long frame, uint64_t new_addr, unsigned int flags,
+ int *page_accessed)
{
struct vcpu *curr = current;
l1_pgentry_t *pl1e, ol1e;
@@ -4069,7 +4072,7 @@ int replace_grant_host_mapping(
}
if ( !new_addr )
- return destroy_grant_va_mapping(addr, frame, curr);
+ return destroy_grant_va_mapping(addr, frame, curr, page_accessed);
pl1e = guest_map_l1e(curr, new_addr, &gl1mfn);
if ( !pl1e )
@@ -4117,7 +4120,7 @@ int replace_grant_host_mapping(
put_page(l1pg);
guest_unmap_l1e(curr, pl1e);
- rc = replace_grant_va_mapping(addr, frame, ol1e, curr);
+ rc = replace_grant_va_mapping(addr, frame, ol1e, curr, page_accessed);
if ( rc && !paging_mode_refcounts(curr->domain) )
put_page_from_l1e(ol1e, curr->domain);
diff -r 3e0ee081c616 -r 38f320029371 xen/common/grant_table.c
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -73,6 +73,7 @@ struct gnttab_unmap_common {
/* Return */
int16_t status;
+ int page_accessed;
/* Shared state beteen *_unmap and *_unmap_complete */
u16 flags;
@@ -81,6 +82,12 @@ struct gnttab_unmap_common {
struct domain *rd;
};
+#ifndef NDEBUG
+atomic_t grant_unmap_tlb_flush_avoided;
+atomic_t grant_unmap_tlb_flush_done;
+atomic_t grant_unmap_operations;
+#endif
+
/* Number of unmap operations that are done between each tlb flush */
#define GNTTAB_UNMAP_BATCH_SIZE 32
@@ -844,6 +851,7 @@ static void
ld = current->domain;
lgt = ld->grant_table;
+ op->page_accessed = 0;
op->frame = (unsigned long)(op->dev_bus_addr >> PAGE_SHIFT);
if ( unlikely(op->handle >= lgt->maptrack_limit) )
@@ -924,7 +932,7 @@ static void
{
if ( (rc = replace_grant_host_mapping(op->host_addr,
op->frame, op->new_addr,
- op->flags)) < 0 )
+ op->flags, &op->page_accessed))
< 0 )
goto unmap_out;
ASSERT(act->pin & (GNTPIN_hstw_mask | GNTPIN_hstr_mask));
@@ -1082,20 +1090,25 @@ static long
gnttab_unmap_grant_ref(
XEN_GUEST_HANDLE_PARAM(gnttab_unmap_grant_ref_t) uop, unsigned int count)
{
- int i, c, partial_done, done = 0;
+ int i, c, partial_done, done = 0, grant_map_accessed = 0;
struct gnttab_unmap_grant_ref op;
struct gnttab_unmap_common common[GNTTAB_UNMAP_BATCH_SIZE];
+#ifndef NDEBUG
+ atomic_inc(&grant_unmap_operations);
+#endif
while ( count != 0 )
{
c = min(count, (unsigned int)GNTTAB_UNMAP_BATCH_SIZE);
partial_done = 0;
+ grant_map_accessed = 0;
for ( i = 0; i < c; i++ )
{
if ( unlikely(__copy_from_guest(&op, uop, 1)) )
goto fault;
__gnttab_unmap_grant_ref(&op, &(common[i]));
+ grant_map_accessed |= common[i].page_accessed;
++partial_done;
if ( unlikely(__copy_field_to_guest(uop, &op, status)) )
goto fault;
@@ -1103,6 +1116,14 @@ gnttab_unmap_grant_ref(
}
gnttab_flush_tlb(current->domain);
+ if ( grant_map_accessed ) {
+ gnttab_flush_tlb(current->domain);
+#ifndef NDEBUG
+ atomic_inc(&grant_unmap_tlb_flush_done);
+ } else {
+ atomic_inc(&grant_unmap_tlb_flush_avoided);
+#endif
+ }
for ( i = 0; i < partial_done; i++ )
__gnttab_unmap_common_complete(&(common[i]));
@@ -2899,7 +2920,12 @@ static void gnttab_usage_print_all(unsig
printk("%s [ key '%c' pressed\n", __FUNCTION__, key);
for_each_domain ( d )
gnttab_usage_print(d);
+#ifndef NDEBUG
+ printk("Grant unmap tlb flush performed %i\n",
atomic_read(&grant_unmap_tlb_flush_done));
+ printk("Grant unmap tlb flush avoided %i\n",
atomic_read(&grant_unmap_tlb_flush_avoided));
+ printk("Grant unmap operations %i\n",
atomic_read(&grant_unmap_operations));
printk("%s ] done\n", __FUNCTION__);
+#endif
}
static struct keyhandler gnttab_usage_print_all_keyhandler = {
diff -r 3e0ee081c616 -r 38f320029371 xen/include/asm-arm/grant_table.h
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -12,7 +12,7 @@ int create_grant_host_mapping(unsigned l
cache_flags);
#define gnttab_host_mapping_get_page_type(op, d, rd) (0)
int replace_grant_host_mapping(unsigned long gpaddr, unsigned long mfn,
- unsigned long new_gpaddr, unsigned int flags);
+ unsigned long new_gpaddr, unsigned int flags, int *page_accessed);
void gnttab_mark_dirty(struct domain *d, unsigned long l);
#define gnttab_create_status_page(d, t, i) do {} while (0)
#define gnttab_status_gmfn(d, t, i) (0)
diff -r 3e0ee081c616 -r 38f320029371 xen/include/asm-x86/grant_table.h
--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -16,7 +16,8 @@
int create_grant_host_mapping(uint64_t addr, unsigned long frame,
unsigned int flags, unsigned int cache_flags);
int replace_grant_host_mapping(
- uint64_t addr, unsigned long frame, uint64_t new_addr, unsigned int flags);
+ uint64_t addr, unsigned long frame, uint64_t new_addr, unsigned int flags,
+ int *page_accessed);
#define gnttab_create_shared_page(d, t, i) \
do { \
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