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Subject[PATCH v4 5/5] KVM: x86: improve reexecute_instruction
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 | 24 +++++++++++++-----
arch/x86/kvm/x86.c | 50 ++++++++++++++++++++++++--------------
3 files changed, 55 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c431b33..de229e6 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 gfn is used as page table in guest which
+ * is set when fix page fault and used to detect unhandeable
+ * instruction.
+ */
+ bool target_gfn_is_pt;
};

struct kvm_lpage_info {
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 0453fa0..ca1be75 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -507,20 +507,27 @@ out_gpte_changed:
*/
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 *target_gfn_is_pt)
{
int level;
gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
+ bool self_changed = false;
+
+ *target_gfn_is_pt = 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);
+ *target_gfn_is_pt |= !gfn;
+ }

- return false;
+ return self_changed;
}

/*
@@ -548,7 +555,7 @@ 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);

@@ -576,9 +583,12 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
return 0;
}

+ is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
+ &walker, user_fault, &vcpu->arch.target_gfn_is_pt);
+
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 b0a3678..44c6992 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4756,15 +4756,8 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
{
gpa_t gpa = cr2;
+ gfn_t gfn;
pfn_t pfn;
- unsigned int indirect_shadow_pages;
-
- spin_lock(&vcpu->kvm->mmu_lock);
- indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
- spin_unlock(&vcpu->kvm->mmu_lock);
-
- if (!indirect_shadow_pages)
- return false;

if (!vcpu->arch.mmu.direct_map) {
/*
@@ -4781,13 +4774,7 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
return true;
}

- /*
- * if emulation was due to access to shadowed page table
- * and it failed try to unshadow page and re-enter the
- * guest to let CPU execute the instruction.
- */
- if (kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa)))
- return true;
+ gfn = gpa_to_gfn(gpa);

/*
* Do not retry the unhandleable instruction if it faults on the
@@ -4795,13 +4782,38 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
* retry instruction -> write #PF -> emulation fail -> retry
* instruction -> ...
*/
- pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
- if (!is_error_noslot_pfn(pfn)) {
- kvm_release_pfn_clean(pfn);
+ pfn = gfn_to_pfn(vcpu->kvm, gfn);
+
+ /*
+ * If the instruction failed on the error pfn, it can not be fixed,
+ * report the error to userspace.
+ */
+ if (is_error_noslot_pfn(pfn))
+ return false;
+
+ kvm_release_pfn_clean(pfn);
+
+ /* The instructions are well-emulated on direct mmu. */
+ if (vcpu->arch.mmu.direct_map) {
+ unsigned int indirect_shadow_pages;
+
+ spin_lock(&vcpu->kvm->mmu_lock);
+ indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
+ spin_unlock(&vcpu->kvm->mmu_lock);
+
+ if (indirect_shadow_pages)
+ kvm_mmu_unprotect_page(vcpu->kvm, gfn);
+
return true;
}

- return false;
+ kvm_mmu_unprotect_page(vcpu->kvm, gfn);
+
+ /* If the target gfn is used as page table, the fault can
+ * not be avoided by unprotecting shadow page and it will
+ * be reported to userspace.
+ */
+ return !vcpu->arch.target_gfn_is_pt;
}

static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
--
1.7.7.6


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