KVM: Make shadow pte updates atomic

With guest smp, a second vcpu might see partial updates when the first
vcpu services a page fault.  So delay all updates until we have figured
out what the pte should look like.

Note that on i386, this is still not completely atomic as a 64-bit write
will be split into two on a 32-bit machine.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 20 insertions, 17 deletions

diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
index 35f264f346d8..397a4039eaad 100644
--- a/drivers/kvm/paging_tmpl.h
+++ b/drivers/kvm/paging_tmpl.h
@@ -205,11 +205,12 @@ static void FNAME(set_pte_common)(struct kvm_vcpu *vcpu,
 {
         hpa_t paddr;
         int dirty = *gpte & PT_DIRTY_MASK;
-        int was_rmapped = is_rmap_pte(*shadow_pte);
+        u64 spte = *shadow_pte;
+        int was_rmapped = is_rmap_pte(spte);
 
         pgprintk("%s: spte %llx gpte %llx access %llx write_fault %d"
                  " user_fault %d gfn %lx\n",
-                 __FUNCTION__, *shadow_pte, (u64)*gpte, access_bits,
+                 __FUNCTION__, spte, (u64)*gpte, access_bits,
                  write_fault, user_fault, gfn);
 
         if (write_fault && !dirty) {
@@ -218,34 +219,35 @@ static void FNAME(set_pte_common)(struct kvm_vcpu *vcpu,
                 FNAME(mark_pagetable_dirty)(vcpu->kvm, walker);
         }
 
-        *shadow_pte |= *gpte & PT_PTE_COPY_MASK;
-        *shadow_pte |= access_bits << PT_SHADOW_BITS_OFFSET;
+        spte |= *gpte & PT_PTE_COPY_MASK;
+        spte |= access_bits << PT_SHADOW_BITS_OFFSET;
         if (!dirty)
                 access_bits &= ~PT_WRITABLE_MASK;
 
         paddr = gpa_to_hpa(vcpu, gaddr & PT64_BASE_ADDR_MASK);
 
-        *shadow_pte |= PT_PRESENT_MASK;
+        spte |= PT_PRESENT_MASK;
         if (access_bits & PT_USER_MASK)
-                *shadow_pte |= PT_USER_MASK;
+                spte |= PT_USER_MASK;
 
         if (is_error_hpa(paddr)) {
-                *shadow_pte |= gaddr;
-                *shadow_pte |= PT_SHADOW_IO_MARK;
-                *shadow_pte &= ~PT_PRESENT_MASK;
+                spte |= gaddr;
+                spte |= PT_SHADOW_IO_MARK;
+                spte &= ~PT_PRESENT_MASK;
+                *shadow_pte = spte;
                 return;
         }
 
-        *shadow_pte |= paddr;
+        spte |= paddr;
 
-        if (!write_fault && (*shadow_pte & PT_SHADOW_USER_MASK) &&
-            !(*shadow_pte & PT_USER_MASK)) {
+        if (!write_fault && (spte & PT_SHADOW_USER_MASK) &&
+            !(spte & PT_USER_MASK)) {
                 /*
                  * If supervisor write protect is disabled, we shadow kernel
                  * pages as user pages so we can trap the write access.
                  */
-                *shadow_pte |= PT_USER_MASK;
-                *shadow_pte &= ~PT_WRITABLE_MASK;
+                spte |= PT_USER_MASK;
+                spte &= ~PT_WRITABLE_MASK;
                 access_bits &= ~PT_WRITABLE_MASK;
         }
 
@@ -253,7 +255,7 @@ static void FNAME(set_pte_common)(struct kvm_vcpu *vcpu,
             || (write_fault && !is_write_protection(vcpu) && !user_fault)) {
                 struct kvm_mmu_page *shadow;
 
-                *shadow_pte |= PT_WRITABLE_MASK;
+                spte |= PT_WRITABLE_MASK;
                 if (user_fault) {
                         mmu_unshadow(vcpu, gfn);
                         goto unshadowed;
@@ -264,8 +266,8 @@ static void FNAME(set_pte_common)(struct kvm_vcpu *vcpu,
                         pgprintk("%s: found shadow page for %lx, marking ro\n",
                                  __FUNCTION__, gfn);
                         access_bits &= ~PT_WRITABLE_MASK;
-                        if (is_writeble_pte(*shadow_pte)) {
-                                *shadow_pte &= ~PT_WRITABLE_MASK;
+                        if (is_writeble_pte(spte)) {
+                                spte &= ~PT_WRITABLE_MASK;
                                 kvm_arch_ops->tlb_flush(vcpu);
                         }
                         if (write_fault)
@@ -278,6 +280,7 @@ unshadowed:
         if (access_bits & PT_WRITABLE_MASK)
                 mark_page_dirty(vcpu->kvm, gaddr >> PAGE_SHIFT);
 
+        *shadow_pte = spte;
         page_header_update_slot(vcpu->kvm, shadow_pte, gaddr);
         if (!was_rmapped)
                 rmap_add(vcpu, shadow_pte);