KVM: x86/mmu: Cache the access bits of shadowed translations

Splitting huge pages requires allocating/finding shadow pages to replace
the huge page. Shadow pages are keyed, in part, off the guest access
permissions they are shadowing. For fully direct MMUs, there is no
shadowing so the access bits in the shadow page role are always ACC_ALL.
But during shadow paging, the guest can enforce whatever access
permissions it wants.

In particular, eager page splitting needs to know the permissions to use
for the subpages, but KVM cannot retrieve them from the guest page
tables because eager page splitting does not have a vCPU.  Fortunately,
the guest access permissions are easy to cache whenever page faults or
FNAME(sync_page) update the shadow page tables; this is an extension of
the existing cache of the shadowed GFNs in the gfns array of the shadow
page.  The access bits only take up 3 bits, which leaves 61 bits left
over for gfns, which is more than enough.

Now that the gfns array caches more information than just GFNs, rename
it to shadowed_translation.

While here, preemptively fix up the WARN_ON() that detects gfn
mismatches in direct SPs. The WARN_ON() was paired with a
pr_err_ratelimited(), which means that users could sometimes see the
WARN without the accompanying error message. Fix this by outputting the
error message as part of the WARN splat, and opportunistically make
them WARN_ONCE() because if these ever fire, they are all but guaranteed
to fire a lot and will bring down the kernel.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-18-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index bb9d12ac0db3..ae2d660e2dab 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -67,8 +67,21 @@ struct kvm_mmu_page {
 	gfn_t gfn;
 
 	u64 *spt;
-	/* hold the gfn of each spte inside spt */
-	gfn_t *gfns;
+
+	/*
+	 * Stores the result of the guest translation being shadowed by each
+	 * SPTE.  KVM shadows two types of guest translations: nGPA -> GPA
+	 * (shadow EPT/NPT) and GVA -> GPA (traditional shadow paging). In both
+	 * cases the result of the translation is a GPA and a set of access
+	 * constraints.
+	 *
+	 * The GFN is stored in the upper bits (PAGE_SHIFT) and the shadowed
+	 * access permissions are stored in the lower bits. Note, for
+	 * convenience and uniformity across guests, the access permissions are
+	 * stored in KVM format (e.g.  ACC_EXEC_MASK) not the raw guest format.
+	 */
+	u64 *shadowed_translation;
+
 	/* Currently serving as active root */
 	union {
 		int root_count;