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linux/arch/x86/kvm/paging_tmpl.h
Avi Kivity f7d9c7b7b9 KVM: MMU: Fix race when instantiating a shadow pte 
For improved concurrency, the guest walk is performed concurrently with other
vcpus.  This means that we need to revalidate the guest ptes once we have
write-protected the guest page tables, at which point they can no longer be
modified.

The current code attempts to avoid this check if the shadow page table is not
new, on the assumption that if it has existed before, the guest could not have
modified the pte without the shadow lock.  However the assumption is incorrect,
as the racing vcpu could have modified the pte, then instantiated the shadow
page, before our vcpu regains control:

  vcpu0        vcpu1

  fault
  walk pte

               modify pte
               fault in same pagetable
               instantiate shadow page

  lookup shadow page
  conclude it is old
  instantiate spte based on stale guest pte

We could do something clever with generation counters, but a test run by
Marcelo suggests this is unnecessary and we can just do the revalidation
unconditionally.  The pte will be in the processor cache and the check can
be quite fast.

Signed-off-by: Avi Kivity <avi@qumranet.com>
2008-03-04 15:19:49 +02:00

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/*
* Kernel-based Virtual Machine driver for Linux
*
* This module enables machines with Intel VT-x extensions to run virtual
* machines without emulation or binary translation.
*
* MMU support
*
* Copyright (C) 2006 Qumranet, Inc.
*
* Authors:
* Yaniv Kamay <yaniv@qumranet.com>
* Avi Kivity <avi@qumranet.com>
*
* This work is licensed under the terms of the GNU GPL, version 2. See
* the COPYING file in the top-level directory.
*
*/
/*
* We need the mmu code to access both 32-bit and 64-bit guest ptes,
* so the code in this file is compiled twice, once per pte size.
*/
#if PTTYPE == 64
#define pt_element_t u64
#define guest_walker guest_walker64
#define FNAME(name) paging##64_##name
#define PT_BASE_ADDR_MASK PT64_BASE_ADDR_MASK
#define PT_DIR_BASE_ADDR_MASK PT64_DIR_BASE_ADDR_MASK
#define PT_INDEX(addr, level) PT64_INDEX(addr, level)
#define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
#define PT_LEVEL_MASK(level) PT64_LEVEL_MASK(level)
#define PT_LEVEL_BITS PT64_LEVEL_BITS
#ifdef CONFIG_X86_64
#define PT_MAX_FULL_LEVELS 4
#define CMPXCHG cmpxchg
#else
#define CMPXCHG cmpxchg64
#define PT_MAX_FULL_LEVELS 2
#endif
#elif PTTYPE == 32
#define pt_element_t u32
#define guest_walker guest_walker32
#define FNAME(name) paging##32_##name
#define PT_BASE_ADDR_MASK PT32_BASE_ADDR_MASK
#define PT_DIR_BASE_ADDR_MASK PT32_DIR_BASE_ADDR_MASK
#define PT_INDEX(addr, level) PT32_INDEX(addr, level)
#define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
#define PT_LEVEL_MASK(level) PT32_LEVEL_MASK(level)
#define PT_LEVEL_BITS PT32_LEVEL_BITS
#define PT_MAX_FULL_LEVELS 2
#define CMPXCHG cmpxchg
#else
#error Invalid PTTYPE value
#endif
#define gpte_to_gfn FNAME(gpte_to_gfn)
#define gpte_to_gfn_pde FNAME(gpte_to_gfn_pde)
/*
* The guest_walker structure emulates the behavior of the hardware page
* table walker.
*/
struct guest_walker {
int level;
gfn_t table_gfn[PT_MAX_FULL_LEVELS];
pt_element_t ptes[PT_MAX_FULL_LEVELS];
gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
unsigned pt_access;
unsigned pte_access;
gfn_t gfn;
u32 error_code;
};
static gfn_t gpte_to_gfn(pt_element_t gpte)
{
return (gpte & PT_BASE_ADDR_MASK) >> PAGE_SHIFT;
}
static gfn_t gpte_to_gfn_pde(pt_element_t gpte)
{
return (gpte & PT_DIR_BASE_ADDR_MASK) >> PAGE_SHIFT;
}
static bool FNAME(cmpxchg_gpte)(struct kvm *kvm,
gfn_t table_gfn, unsigned index,
pt_element_t orig_pte, pt_element_t new_pte)
{
pt_element_t ret;
pt_element_t *table;
struct page *page;
down_read(&current->mm->mmap_sem);
page = gfn_to_page(kvm, table_gfn);
up_read(&current->mm->mmap_sem);
table = kmap_atomic(page, KM_USER0);
ret = CMPXCHG(&table[index], orig_pte, new_pte);
kunmap_atomic(table, KM_USER0);
kvm_release_page_dirty(page);
return (ret != orig_pte);
}
static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte)
{
unsigned access;
access = (gpte & (PT_WRITABLE_MASK | PT_USER_MASK)) | ACC_EXEC_MASK;
#if PTTYPE == 64
if (is_nx(vcpu))
access &= ~(gpte >> PT64_NX_SHIFT);
#endif
return access;
}
/*
* Fetch a guest pte for a guest virtual address
*/
static int FNAME(walk_addr)(struct guest_walker *walker,
struct kvm_vcpu *vcpu, gva_t addr,
int write_fault, int user_fault, int fetch_fault)
{
pt_element_t pte;
gfn_t table_gfn;
unsigned index, pt_access, pte_access;
gpa_t pte_gpa;
pgprintk("%s: addr %lx\n", __FUNCTION__, addr);
walk:
walker->level = vcpu->arch.mmu.root_level;
pte = vcpu->arch.cr3;
#if PTTYPE == 64
if (!is_long_mode(vcpu)) {
pte = vcpu->arch.pdptrs[(addr >> 30) & 3];
if (!is_present_pte(pte))
goto not_present;
--walker->level;
}
#endif
ASSERT((!is_long_mode(vcpu) && is_pae(vcpu)) ||
(vcpu->arch.cr3 & CR3_NONPAE_RESERVED_BITS) == 0);
pt_access = ACC_ALL;
for (;;) {
index = PT_INDEX(addr, walker->level);
table_gfn = gpte_to_gfn(pte);
pte_gpa = gfn_to_gpa(table_gfn);
pte_gpa += index * sizeof(pt_element_t);
walker->table_gfn[walker->level - 1] = table_gfn;
walker->pte_gpa[walker->level - 1] = pte_gpa;
pgprintk("%s: table_gfn[%d] %lx\n", __FUNCTION__,
walker->level - 1, table_gfn);
kvm_read_guest(vcpu->kvm, pte_gpa, &pte, sizeof(pte));
if (!is_present_pte(pte))
goto not_present;
if (write_fault && !is_writeble_pte(pte))
if (user_fault || is_write_protection(vcpu))
goto access_error;
if (user_fault && !(pte & PT_USER_MASK))
goto access_error;
#if PTTYPE == 64
if (fetch_fault && is_nx(vcpu) && (pte & PT64_NX_MASK))
goto access_error;
#endif
if (!(pte & PT_ACCESSED_MASK)) {
mark_page_dirty(vcpu->kvm, table_gfn);
if (FNAME(cmpxchg_gpte)(vcpu->kvm, table_gfn,
index, pte, pte|PT_ACCESSED_MASK))
goto walk;
pte |= PT_ACCESSED_MASK;
}
pte_access = pt_access & FNAME(gpte_access)(vcpu, pte);
walker->ptes[walker->level - 1] = pte;
if (walker->level == PT_PAGE_TABLE_LEVEL) {
walker->gfn = gpte_to_gfn(pte);
break;
}
if (walker->level == PT_DIRECTORY_LEVEL
&& (pte & PT_PAGE_SIZE_MASK)
&& (PTTYPE == 64 || is_pse(vcpu))) {
walker->gfn = gpte_to_gfn_pde(pte);
walker->gfn += PT_INDEX(addr, PT_PAGE_TABLE_LEVEL);
if (PTTYPE == 32 && is_cpuid_PSE36())
walker->gfn += pse36_gfn_delta(pte);
break;
}
pt_access = pte_access;
--walker->level;
}
if (write_fault && !is_dirty_pte(pte)) {
bool ret;
mark_page_dirty(vcpu->kvm, table_gfn);
ret = FNAME(cmpxchg_gpte)(vcpu->kvm, table_gfn, index, pte,
pte|PT_DIRTY_MASK);
if (ret)
goto walk;
pte |= PT_DIRTY_MASK;
kvm_mmu_pte_write(vcpu, pte_gpa, (u8 *)&pte, sizeof(pte));
walker->ptes[walker->level - 1] = pte;
}
walker->pt_access = pt_access;
walker->pte_access = pte_access;
pgprintk("%s: pte %llx pte_access %x pt_access %x\n",
__FUNCTION__, (u64)pte, pt_access, pte_access);
return 1;
not_present:
walker->error_code = 0;
goto err;
access_error:
walker->error_code = PFERR_PRESENT_MASK;
err:
if (write_fault)
walker->error_code |= PFERR_WRITE_MASK;
if (user_fault)
walker->error_code |= PFERR_USER_MASK;
if (fetch_fault)
walker->error_code |= PFERR_FETCH_MASK;
return 0;
}
static void FNAME(update_pte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *page,
u64 *spte, const void *pte, int bytes,
int offset_in_pte)
{
pt_element_t gpte;
unsigned pte_access;
struct page *npage;
gpte = *(const pt_element_t *)pte;
if (~gpte & (PT_PRESENT_MASK | PT_ACCESSED_MASK)) {
if (!offset_in_pte && !is_present_pte(gpte))
set_shadow_pte(spte, shadow_notrap_nonpresent_pte);
return;
}
if (bytes < sizeof(pt_element_t))
return;
pgprintk("%s: gpte %llx spte %p\n", __FUNCTION__, (u64)gpte, spte);
pte_access = page->role.access & FNAME(gpte_access)(vcpu, gpte);
if (gpte_to_gfn(gpte) != vcpu->arch.update_pte.gfn)
return;
npage = vcpu->arch.update_pte.page;
if (!npage)
return;
get_page(npage);
mmu_set_spte(vcpu, spte, page->role.access, pte_access, 0, 0,
gpte & PT_DIRTY_MASK, NULL, gpte_to_gfn(gpte), npage);
}
/*
* Fetch a shadow pte for a specific level in the paging hierarchy.
*/
static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
struct guest_walker *walker,
int user_fault, int write_fault, int *ptwrite,
struct page *page)
{
hpa_t shadow_addr;
int level;
u64 *shadow_ent;
unsigned access = walker->pt_access;
if (!is_present_pte(walker->ptes[walker->level - 1]))
return NULL;
shadow_addr = vcpu->arch.mmu.root_hpa;
level = vcpu->arch.mmu.shadow_root_level;
if (level == PT32E_ROOT_LEVEL) {
shadow_addr = vcpu->arch.mmu.pae_root[(addr >> 30) & 3];
shadow_addr &= PT64_BASE_ADDR_MASK;
--level;
}
for (; ; level--) {
u32 index = SHADOW_PT_INDEX(addr, level);
struct kvm_mmu_page *shadow_page;
u64 shadow_pte;
int metaphysical;
gfn_t table_gfn;
shadow_ent = ((u64 *)__va(shadow_addr)) + index;
if (level == PT_PAGE_TABLE_LEVEL)
break;
if (is_shadow_present_pte(*shadow_ent)) {
shadow_addr = *shadow_ent & PT64_BASE_ADDR_MASK;
continue;
}
if (level - 1 == PT_PAGE_TABLE_LEVEL
&& walker->level == PT_DIRECTORY_LEVEL) {
metaphysical = 1;
if (!is_dirty_pte(walker->ptes[level - 1]))
access &= ~ACC_WRITE_MASK;
table_gfn = gpte_to_gfn(walker->ptes[level - 1]);
} else {
metaphysical = 0;
table_gfn = walker->table_gfn[level - 2];
}
shadow_page = kvm_mmu_get_page(vcpu, table_gfn, addr, level-1,
metaphysical, access,
shadow_ent);
if (!metaphysical) {
int r;
pt_element_t curr_pte;
r = kvm_read_guest_atomic(vcpu->kvm,
walker->pte_gpa[level - 2],
&curr_pte, sizeof(curr_pte));
if (r || curr_pte != walker->ptes[level - 2]) {
kvm_release_page_clean(page);
return NULL;
}
}
shadow_addr = __pa(shadow_page->spt);
shadow_pte = shadow_addr | PT_PRESENT_MASK | PT_ACCESSED_MASK
| PT_WRITABLE_MASK | PT_USER_MASK;
*shadow_ent = shadow_pte;
}
mmu_set_spte(vcpu, shadow_ent, access, walker->pte_access & access,
user_fault, write_fault,
walker->ptes[walker->level-1] & PT_DIRTY_MASK,
ptwrite, walker->gfn, page);
return shadow_ent;
}
/*
* Page fault handler. There are several causes for a page fault:
* - there is no shadow pte for the guest pte
* - write access through a shadow pte marked read only so that we can set
* the dirty bit
* - write access to a shadow pte marked read only so we can update the page
* dirty bitmap, when userspace requests it
* - mmio access; in this case we will never install a present shadow pte
* - normal guest page fault due to the guest pte marked not present, not
* writable, or not executable
*
* Returns: 1 if we need to emulate the instruction, 0 otherwise, or
* a negative value on error.
*/
static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr,
u32 error_code)
{
int write_fault = error_code & PFERR_WRITE_MASK;
int user_fault = error_code & PFERR_USER_MASK;
int fetch_fault = error_code & PFERR_FETCH_MASK;
struct guest_walker walker;
u64 *shadow_pte;
int write_pt = 0;
int r;
struct page *page;
pgprintk("%s: addr %lx err %x\n", __FUNCTION__, addr, error_code);
kvm_mmu_audit(vcpu, "pre page fault");
r = mmu_topup_memory_caches(vcpu);
if (r)
return r;
down_read(&vcpu->kvm->slots_lock);
/*
* Look up the shadow pte for the faulting address.
*/
r = FNAME(walk_addr)(&walker, vcpu, addr, write_fault, user_fault,
fetch_fault);
/*
* The page is not mapped by the guest. Let the guest handle it.
*/
if (!r) {
pgprintk("%s: guest page fault\n", __FUNCTION__);
inject_page_fault(vcpu, addr, walker.error_code);
vcpu->arch.last_pt_write_count = 0; /* reset fork detector */
up_read(&vcpu->kvm->slots_lock);
return 0;
}
down_read(&current->mm->mmap_sem);
page = gfn_to_page(vcpu->kvm, walker.gfn);
up_read(&current->mm->mmap_sem);
spin_lock(&vcpu->kvm->mmu_lock);
kvm_mmu_free_some_pages(vcpu);
shadow_pte = FNAME(fetch)(vcpu, addr, &walker, user_fault, write_fault,
&write_pt, page);
pgprintk("%s: shadow pte %p %llx ptwrite %d\n", __FUNCTION__,
shadow_pte, *shadow_pte, write_pt);
if (!write_pt)
vcpu->arch.last_pt_write_count = 0; /* reset fork detector */
/*
* mmio: emulate if accessible, otherwise its a guest fault.
*/
if (shadow_pte && is_io_pte(*shadow_pte)) {
spin_unlock(&vcpu->kvm->mmu_lock);
up_read(&vcpu->kvm->slots_lock);
return 1;
}
++vcpu->stat.pf_fixed;
kvm_mmu_audit(vcpu, "post page fault (fixed)");
spin_unlock(&vcpu->kvm->mmu_lock);
up_read(&vcpu->kvm->slots_lock);
return write_pt;
}
static gpa_t FNAME(gva_to_gpa)(struct kvm_vcpu *vcpu, gva_t vaddr)
{
struct guest_walker walker;
gpa_t gpa = UNMAPPED_GVA;
int r;
r = FNAME(walk_addr)(&walker, vcpu, vaddr, 0, 0, 0);
if (r) {
gpa = gfn_to_gpa(walker.gfn);
gpa |= vaddr & ~PAGE_MASK;
}
return gpa;
}
static void FNAME(prefetch_page)(struct kvm_vcpu *vcpu,
struct kvm_mmu_page *sp)
{
int i, offset = 0, r = 0;
pt_element_t pt;
if (sp->role.metaphysical
|| (PTTYPE == 32 && sp->role.level > PT_PAGE_TABLE_LEVEL)) {
nonpaging_prefetch_page(vcpu, sp);
return;
}
if (PTTYPE == 32)
offset = sp->role.quadrant << PT64_LEVEL_BITS;
for (i = 0; i < PT64_ENT_PER_PAGE; ++i) {
gpa_t pte_gpa = gfn_to_gpa(sp->gfn);
pte_gpa += (i+offset) * sizeof(pt_element_t);
r = kvm_read_guest_atomic(vcpu->kvm, pte_gpa, &pt,
sizeof(pt_element_t));
if (r || is_present_pte(pt))
sp->spt[i] = shadow_trap_nonpresent_pte;
else
sp->spt[i] = shadow_notrap_nonpresent_pte;
}
}
#undef pt_element_t
#undef guest_walker
#undef FNAME
#undef PT_BASE_ADDR_MASK
#undef PT_INDEX
#undef SHADOW_PT_INDEX
#undef PT_LEVEL_MASK
#undef PT_DIR_BASE_ADDR_MASK
#undef PT_LEVEL_BITS
#undef PT_MAX_FULL_LEVELS
#undef gpte_to_gfn
#undef gpte_to_gfn_pde
#undef CMPXCHG
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