[PATCH v6 3/3] KVM: x86: improve reexecute_instruction

Xiao Guangrong
2013-01-13T15:49:53Z

The current reexecute_instruction can not well detect the failed instruction
emulation. It allows guest to retry all the instructions except it accesses
on error pfn

For example, some cases are nested-write-protect - if the page we want to
write is used as PDE but it chains to itself. Under this case, we should
stop the emulation and report the case to userspace

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/include/asm/kvm_host.h | 7 +++++++
arch/x86/kvm/paging_tmpl.h | 27 ++++++++++++++++++++-------
arch/x86/kvm/x86.c | 22 ++++++++++++++++++----
3 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c431b33..d6ab8d2 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -502,6 +502,13 @@ struct kvm_vcpu_arch {
u64 msr_val;
struct gfn_to_hva_cache data;
} pv_eoi;
+
+ /*
+ * Indicate whether the access faults on its page table in guest
+ * which is set when fix page fault and used to detect unhandeable
+ * instruction.
+ */
+ bool write_fault_to_shadow_pgtable;
};

struct kvm_lpage_info {
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 3d1a352..ca69dcc 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -497,26 +497,34 @@ out_gpte_changed:
* created when kvm establishes shadow page table that stop kvm using large
* page size. Do it early can avoid unnecessary #PF and emulation.
*
+ * @write_fault_to_shadow_pgtable will return true if the fault gfn is
+ * currently used as its page table.
+ *
* Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
* since the PDPT is always shadowed, that means, we can not use large page
* size to map the gfn which is used as PDPT.
*/
static bool
FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
- struct guest_walker *walker, int user_fault)
+ struct guest_walker *walker, int user_fault,
+ bool *write_fault_to_shadow_pgtable)
{
int level;
gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
+ bool self_changed = false;

if (!(walker->pte_access & ACC_WRITE_MASK ||
(!is_write_protection(vcpu) && !user_fault)))
return false;

- for (level = walker->level; level <= walker->max_level; level++)
- if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))
- return true;
+ for (level = walker->level; level <= walker->max_level; level++) {
+ gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
+
+ self_changed |= !(gfn & mask);
+ *write_fault_to_shadow_pgtable |= !gfn;
+ }

- return false;
+ return self_changed;
}

/*
@@ -544,7 +552,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
int level = PT_PAGE_TABLE_LEVEL;
int force_pt_level;
unsigned long mmu_seq;
- bool map_writable;
+ bool map_writable, is_self_change_mapping;

pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);

@@ -572,9 +580,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
return 0;
}

+ vcpu->arch.write_fault_to_shadow_pgtable = false;
+
+ is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
+ &walker, user_fault, &vcpu->arch.write_fault_to_shadow_pgtable);
+
if (walker.level >= PT_DIRECTORY_LEVEL)
force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn)
- || FNAME(is_self_change_mapping)(vcpu, &walker, user_fault);
+ || is_self_change_mapping;
else
force_pt_level = 1;
if (!force_pt_level) {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6f13e03..139a5f8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4753,7 +4753,8 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
return r;
}

-static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
+static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2,
+ bool write_fault_to_shadow_pgtable)
{
gpa_t gpa = cr2;
pfn_t pfn;
@@ -4810,7 +4811,13 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
* guest to let CPU execute the instruction.
*/
kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
- return true;
+
+ /*
+ * If the access faults on its page table, it can not
+ * be fixed by unprotecting shadow page and it should
+ * be reported to userspace.
+ */
+ return !write_fault_to_shadow_pgtable;
}

static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
@@ -4869,7 +4876,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
int r;
struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
bool writeback = true;
+ bool write_fault_to_spt = vcpu->arch.write_fault_to_shadow_pgtable;

+ /*
+ * Clear write_fault_to_shadow_pgtable here to ensure it is
+ * never reused.
+ */
+ vcpu->arch.write_fault_to_shadow_pgtable = false;
kvm_clear_exception_queue(vcpu);

if (!(emulation_type & EMULTYPE_NO_DECODE)) {
@@ -4888,7 +4901,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
if (r != EMULATION_OK) {
if (emulation_type & EMULTYPE_TRAP_UD)
return EMULATE_FAIL;
- if (reexecute_instruction(vcpu, cr2))
+ if (reexecute_instruction(vcpu, cr2,
+ write_fault_to_spt))
return EMULATE_DONE;
if (emulation_type & EMULTYPE_SKIP)
return EMULATE_FAIL;
@@ -4918,7 +4932,7 @@ restart:
return EMULATE_DONE;

if (r == EMULATION_FAILED) {
- if (reexecute_instruction(vcpu, cr2))
+ if (reexecute_instruction(vcpu, cr2, write_fault_to_spt))
return EMULATE_DONE;

return handle_emulation_failure(vcpu);
--
1.7.7.6