[RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

Sean Christopherson seanjc at google.com
Thu Sep 14 11:55:05 AEST 2023

From: Chao Peng <chao.p.peng at linux.intel.com>

Add a new KVM exit type to allow userspace to handle memory faults that
KVM cannot resolve, but that userspace *may* be able to handle (without
terminating the guest).

KVM will initially use KVM_EXIT_MEMORY_FAULT to report implicit
conversions between private and shared memory.  With guest private memory,
there will be  two kind of memory conversions:

  - explicit conversion: happens when the guest explicitly calls into KVM
    to map a range (as private or shared)

  - implicit conversion: happens when the guest attempts to access a gfn
    that is configured in the "wrong" state (private vs. shared)

On x86 (first architecture to support guest private memory), explicit
conversions will be reported via KVM_EXIT_HYPERCALL+KVM_HC_MAP_GPA_RANGE,
but reporting KVM_EXIT_HYPERCALL for implicit conversions is undesriable
as there is (obviously) no hypercall, and there is no guarantee that the
guest actually intends to convert between private and shared, i.e. what
KVM thinks is an implicit conversion "request" could actually be the
result of a guest code bug.

KVM_EXIT_MEMORY_FAULT will be used to report memory faults that appear to
be implicit conversions.

Place "struct memory_fault" in a second anonymous union so that filling
memory_fault doesn't clobber state from other yet-to-be-fulfilled exits,
and to provide additional information if KVM does NOT ultimately exit to
userspace with KVM_EXIT_MEMORY_FAULT, e.g. if KVM suppresses (or worse,
loses) the exit, as KVM often suppresses exits for memory failures that
occur when accessing paravirt data structures.  The initial usage for
private memory will be all-or-nothing, but other features such as the
proposed "userfault on missing mappings" support will use
KVM_EXIT_MEMORY_FAULT for potentially _all_ guest memory accesses, i.e.
will run afoul of KVM's various quirks.

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.

Note!  To allow for future possibilities where KVM reports
KVM_EXIT_MEMORY_FAULT and fills run->memory_fault on _any_ unresolved
fault, KVM returns "-EFAULT" (-1 with errno == EFAULT from userspace's
perspective), not '0'!  Due to historical baggage within KVM, exiting to
userspace with '0' from deep callstacks, e.g. in emulation paths, is
infeasible as doing so would require a near-complete overhaul of KVM,
whereas KVM already propagates -errno return codes to userspace even when
the -errno originated in a low level helper.

Link: https://lore.kernel.org/all/20230908222905.1321305-5-amoorthy@google.com
Cc: Anish Moorthy <amoorthy at google.com>
Suggested-by: Sean Christopherson <seanjc at google.com>
Co-developed-by: Yu Zhang <yu.c.zhang at linux.intel.com>
Signed-off-by: Yu Zhang <yu.c.zhang at linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng at linux.intel.com>
Co-developed-by: Sean Christopherson <seanjc at google.com>
Signed-off-by: Sean Christopherson <seanjc at google.com>
 Documentation/virt/kvm/api.rst | 24 ++++++++++++++++++++++++
 include/linux/kvm_host.h       | 15 +++++++++++++++
 include/uapi/linux/kvm.h       | 24 ++++++++++++++++++++++++
 3 files changed, 63 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 21a7578142a1..e28a13439a95 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6702,6 +6702,30 @@ array field represents return values. The userspace should update the return
 values of SBI call before resuming the VCPU. For more details on RISC-V SBI
 spec refer, https://github.com/riscv/riscv-sbi-doc.
+		struct {
+			__u64 flags;
+			__u64 gpa;
+			__u64 size;
+		} memory;
+KVM_EXIT_MEMORY_FAULT indicates the vCPU has encountered a memory fault that
+could not be resolved by KVM.  The 'gpa' and 'size' (in bytes) describe the
+guest physical address range [gpa, gpa + size) of the fault.  The 'flags' field
+describes properties of the faulting access that are likely pertinent:
+ - KVM_MEMORY_EXIT_FLAG_PRIVATE - When set, indicates the memory fault occurred
+   on a private memory access.  When clear, indicates the fault occurred on a
+   shared access.
+Note!  KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in that it
+accompanies a return code of '-1', not '0'!  errno will always be set to EFAULT
+or EHWPOISON when KVM exits with KVM_EXIT_MEMORY_FAULT, userspace should assume
+kvm_run.exit_reason is stale/undefined for all other error numbers.
     /* KVM_EXIT_NOTIFY */
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4e741ff27af3..d8c6ce6c8211 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2327,4 +2327,19 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
 /* Max number of entries allowed for each kvm dirty ring */
+static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
+						 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;
+	/* 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;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index bd1abe067f28..d2d913acf0df 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -274,6 +274,7 @@ struct kvm_xen_exit {
 #define KVM_EXIT_RISCV_SBI        35
 #define KVM_EXIT_RISCV_CSR        36
 #define KVM_EXIT_NOTIFY           37
+#define KVM_EXIT_MEMORY_FAULT     38
 /* Emulate instruction failed. */
@@ -541,6 +542,29 @@ struct kvm_run {
 		struct kvm_sync_regs regs;
 		char padding[SYNC_REGS_SIZE_BYTES];
 	} s;
+	/*
+	 * This second exit union holds structs for exit types which may be
+	 * triggered after KVM has already initiated a different exit, or which
+	 * may be ultimately dropped by KVM.
+	 *
+	 * For example, because of limitations in KVM's uAPI, KVM x86 can
+	 * generate a memory fault exit an MMIO exit is initiated (exit_reason
+	 * and kvm_run.mmio are filled).  And conversely, KVM often disables
+	 * paravirt features if a memory fault occurs when accessing paravirt
+	 * data instead of reporting the error to userspace.
+	 */
+	union {
+		struct {
+			__u64 flags;
+			__u64 gpa;
+			__u64 size;
+		} memory_fault;
+		/* Fix the size of the union. */
+		char padding2[256];
+	};

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