KVM: x86/mmu: Rename unsync helper and update related comments

Rename mmu_need_write_protect() to mmu_try_to_unsync_pages() and update
a variety of related, stale comments.  Add several new comments to call
out subtle details, e.g. that upper-level shadow pages are write-tracked,
and that can_unsync is false iff KVM is in the process of synchronizing
pages.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-14-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 92b7ab1..dffa948 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2458,17 +2458,33 @@ static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
        kvm_mmu_mark_parents_unsync(sp);
 }
 
-bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
-                           bool can_unsync)
+/*
+ * Attempt to unsync any shadow pages that can be reached by the specified gfn,
+ * KVM is creating a writable mapping for said gfn.  Returns 0 if all pages
+ * were marked unsync (or if there is no shadow page), -EPERM if the SPTE must
+ * be write-protected.
+ */
+int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
 {
        struct kvm_mmu_page *sp;
 
+       /*
+        * Force write-protection if the page is being tracked.  Note, the page
+        * track machinery is used to write-protect upper-level shadow pages,
+        * i.e. this guards the role.level == 4K assertion below!
+        */
        if (kvm_page_track_is_active(vcpu, gfn, KVM_PAGE_TRACK_WRITE))
-               return true;
+               return -EPERM;
 
+       /*
+        * The page is not write-tracked, mark existing shadow pages unsync
+        * unless KVM is synchronizing an unsync SP (can_unsync = false).  In
+        * that case, KVM must complete emulation of the guest TLB flush before
+        * allowing shadow pages to become unsync (writable by the guest).
+        */
        for_each_gfn_indirect_valid_sp(vcpu->kvm, sp, gfn) {
                if (!can_unsync)
-                       return true;
+                       return -EPERM;
 
                if (sp->unsync)
                        continue;
@@ -2499,8 +2515,8 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
         *                      2.2 Guest issues TLB flush.
         *                          That causes a VM Exit.
         *
-        *                      2.3 kvm_mmu_sync_pages() reads sp->unsync.
-        *                          Since it is false, so it just returns.
+        *                      2.3 Walking of unsync pages sees sp->unsync is
+        *                          false and skips the page.
         *
         *                      2.4 Guest accesses GVA X.
         *                          Since the mapping in the SP was not updated,
@@ -2516,7 +2532,7 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
         */
        smp_wmb();
 
-       return false;
+       return 0;
 }
 
 static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
@@ -3461,8 +3477,8 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
                 * flush strictly after those changes are made. We only need to
                 * ensure that the other CPU sets these flags before any actual
                 * changes to the page tables are made. The comments in
-                * mmu_need_write_protect() describe what could go wrong if this
-                * requirement isn't satisfied.
+                * mmu_try_to_unsync_pages() describe what could go wrong if
+                * this requirement isn't satisfied.
                 */
                if (!smp_load_acquire(&sp->unsync) &&
                    !smp_load_acquire(&sp->unsync_children))

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 18be103..3556729 100644 (file)
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -122,8 +122,7 @@ static inline bool is_nx_huge_page_enabled(void)
        return READ_ONCE(nx_huge_pages);
 }
 
-bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
-                           bool can_unsync);
+int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync);
 
 void kvm_mmu_gfn_disallow_lpage(struct kvm_memory_slot *slot, gfn_t gfn);
 void kvm_mmu_gfn_allow_lpage(struct kvm_memory_slot *slot, gfn_t gfn);

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 8e8e8da..246e61e 100644 (file)
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -147,13 +147,19 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
                /*
                 * Optimization: for pte sync, if spte was writable the hash
                 * lookup is unnecessary (and expensive). Write protection
-                * is responsibility of mmu_get_page / kvm_sync_page.
+                * is responsibility of kvm_mmu_get_page / kvm_mmu_sync_roots.
                 * Same reasoning can be applied to dirty page accounting.
                 */
                if (!can_unsync && is_writable_pte(old_spte))
                        goto out;
 
-               if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {
+               /*
+                * Unsync shadow pages that are reachable by the new, writable
+                * SPTE.  Write-protect the SPTE if the page can't be unsync'd,
+                * e.g. it's write-tracked (upper-level SPs) or has one or more
+                * shadow pages and unsync'ing pages is not allowed.
+                */
+               if (mmu_try_to_unsync_pages(vcpu, gfn, can_unsync)) {
                        pgprintk("%s: found shadow page for %llx, marking ro\n",
                                 __func__, gfn);
                        ret |= SET_SPTE_WRITE_PROTECTED_PT;