[PATCH 09/34] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

Paolo Bonzini pbonzini at redhat.com
Mon Nov 6 03:30:12 AEDT 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.

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.

Report the gpa+size instead of a single gfn even though the initial usage
is expected to always report single pages.  It's entirely possible, likely
even, that KVM will someday support sub-page granularity faults, e.g.
Intel's sub-page protection feature allows for additional protections at
128-byte granularity.

Link: https://lore.kernel.org/all/20230908222905.1321305-5-amoorthy@google.com
Link: https://lore.kernel.org/all/ZQ3AmLO2SYv3DszH@google.com
Cc: Anish Moorthy <amoorthy at google.com>
Cc: David Matlack <dmatlack 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>
Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>
Message-Id: <20231027182217.3615211-10-seanjc at google.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
 Documentation/virt/kvm/api.rst | 41 ++++++++++++++++++++++++++++++++++
 arch/x86/kvm/x86.c             |  1 +
 include/linux/kvm_host.h       | 11 +++++++++
 include/uapi/linux/kvm.h       |  8 +++++++
 4 files changed, 61 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index bdea1423c5f8..481fb0e2ce90 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6846,6 +6846,26 @@ 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_fault;
+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.
+Currently, no flags are defined.
+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 */
@@ -7880,6 +7900,27 @@ This capability is aimed to mitigate the threat that malicious VMs can
 cause CPU stuck (due to event windows don't open up) and make the CPU
 unavailable to host or other VMs.
+:Architectures: x86
+:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP.
+The presence of this capability indicates that KVM_RUN will fill
+kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if
+there is a valid memslot but no backing VMA for the corresponding host virtual
+The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns
+an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set
+Note: Userspaces which attempt to resolve memory faults so that they can retry
+KVM_RUN are encouraged to guard against repeatedly receiving the same
+error/annotated fault.
+See KVM_EXIT_MEMORY_FAULT for more information.
 8. Other capabilities.
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7b389f27dffc..8f9d8939b63b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4625,6 +4625,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = 1;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4e741ff27af3..96aa930536b1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2327,4 +2327,15 @@ 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)
+	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. */
+	vcpu->run->memory_fault.flags = 0;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 308cc70bd6ab..59010a685007 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -275,6 +275,7 @@ struct kvm_xen_exit {
 #define KVM_EXIT_RISCV_CSR        36
 #define KVM_EXIT_NOTIFY           37
+#define KVM_EXIT_MEMORY_FAULT     39
 /* Emulate instruction failed. */
@@ -528,6 +529,12 @@ struct kvm_run {
 			__u32 flags;
 		} notify;
+		struct {
+			__u64 flags;
+			__u64 gpa;
+			__u64 size;
+		} memory_fault;
 		/* Fix the size of the union. */
 		char padding[256];
@@ -1212,6 +1219,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_USER_MEMORY2 231

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