KVM: x86/mmu: Handle page fault for private memory

Add support for resolving page faults on guest private memory for VMs
that differentiate between "shared" and "private" memory.  For such VMs,
KVM_MEM_GUEST_MEMFD memslots can include both fd-based private memory and
hva-based shared memory, and KVM needs to map in the "correct" variant,
i.e. KVM needs to map the gfn shared/private as appropriate based on the
current state of the gfn's KVM_MEMORY_ATTRIBUTE_PRIVATE flag.

For AMD's SEV-SNP and Intel's TDX, the guest effectively gets to request
shared vs. private via a bit in the guest page tables, i.e. what the guest
wants may conflict with the current memory attributes.  To support such
"implicit" conversion requests, exit to user with KVM_EXIT_MEMORY_FAULT
to forward the request to userspace.  Add a new flag for memory faults,
KVM_MEMORY_EXIT_FLAG_PRIVATE, to communicate whether the guest wants to
map memory as shared vs. private.

Like KVM_MEMORY_ATTRIBUTE_PRIVATE, use bit 3 for flagging private memory
so that KVM can use bits 0-2 for capturing RWX behavior if/when userspace
needs such information, e.g. a likely user of KVM_EXIT_MEMORY_FAULT is to
exit on missing mappings when handling guest page fault VM-Exits.  In
that case, userspace will want to know RWX information in order to
correctly/precisely resolve the fault.

Note, private memory *must* be backed by guest_memfd, i.e. shared mappings
always come from the host userspace page tables, and private mappings
always come from a guest_memfd instance.

Co-developed-by: Yu Zhang <[email protected]>
Signed-off-by: Yu Zhang <[email protected]>
Signed-off-by: Chao Peng <[email protected]>
Co-developed-by: Sean Christopherson <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Fuad Tabba <[email protected]>
Tested-by: Fuad Tabba <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

2 files changed, 7 insertions, 2 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index a6de526c0426..67dfd4d79529 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2357,14 +2357,18 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
 #define  KVM_DIRTY_RING_MAX_ENTRIES  65536
 
 static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
-						 gpa_t gpa, gpa_t size)
+						 gpa_t gpa, gpa_t size,
+						 bool is_write, bool is_exec,
+						 bool is_private)
 {
 	vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
 	vcpu->run->memory_fault.gpa = gpa;
 	vcpu->run->memory_fault.size = size;
 
-	/* Flags are not (yet) defined or communicated to userspace. */
+	/* RWX flags are not (yet) defined or communicated to userspace. */
 	vcpu->run->memory_fault.flags = 0;
+	if (is_private)
+		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
 }
 
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 2802d10aa88c..8eb10f560c69 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -535,6 +535,7 @@ struct kvm_run {
 		} notify;
 		/* KVM_EXIT_MEMORY_FAULT */
 		struct {
+#define KVM_MEMORY_EXIT_FLAG_PRIVATE	(1ULL << 3)
 			__u64 flags;
 			__u64 gpa;
 			__u64 size;