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Subject: [PATCH RFC v9 35/51] KVM: SVM: Add KVM_EXIT_VMGEXIT
Date:   Sun, 11 Jun 2023 23:25:43 -0500
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For private memslots, GHCB page state change requests will be forwarded
to userspace for processing. Define a new KVM_EXIT_VMGEXIT for exits of
this type, as well as other potential userspace handling for VMGEXITs in
the future.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 Documentation/virt/kvm/api.rst | 34 ++++++++++++++++++++++++++++++++++
 include/uapi/linux/kvm.h       |  6 ++++++
 2 files changed, 40 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index df37aa11512d..028fd3fa50a7 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6780,6 +6780,40 @@ Please note that the kernel is allowed to use the kvm_run structure as the
 primary storage for certain register types. Therefore, the kernel may use the
 values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
 
+::
+
+		/* KVM_EXIT_VMGEXIT */
+		struct {
+			__u64 ghcb_msr; /* GHCB MSR contents */
+			__u64 ret;      /* user -> kernel return value */
+		} memory;
+
+If exit reason is KVM_EXIT_VMGEXIT then it indicates that an SEV-SNP guest has
+issued a VMGEXIT instruction (as documented by the AMD Architecture
+Programmer's Manual (APM)) to the hypervisor that needs to be serviced by
+userspace. This is generally handled via the Guest-Hypervisor Communication
+Block (GHCB) specification. The value of 'ghcb_msr' will be the contents of
+the GHCB MSR register at the time of the VMGEXIT, which can either be the GPA
+of the GHCB page for page-based GHCB requests, or an encoding of an MSR-based
+GHCB request. The mechanism to distinguish between these two and determine the
+type of request is the same as what is documented in the GHCB specification.
+
+Not all VMGEXITs or GHCB requests will be forwarded to userspace. Currently
+this will only be the case for "SNP Page State Change" requests (PSCs), and
+only for the subset of these which involve actual shared <-> private
+transition. Userspace is expected to process these requests in accordance
+with the GHCB specification and issue KVM_SET_MEMORY_ATTRIBUTE ioctls to
+perform the shared/private transitions.
+
+GHCB page-based PSC requests require returning a 64-bit return value to the
+guest via the SW_EXITINFO2 field of the vCPU's VMCB structure, as documented
+in the GHCB. Userspace must set 'ret' to what the GHCB specification documents
+the SW_EXITINFO2 VMCB field should be set to after processing a PSC request.
+
+For MSR-based PSC requests, userspace must set the value of 'ghcb_msr' to be
+the same as what the GHCB specification documents the actual GHCB MSR register
+should be set to after processing a PSC request.
+
 
 6. Capabilities that can be enabled on vCPUs
 ============================================
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 1fb6a6615d09..175b958f103f 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -279,6 +279,7 @@ struct kvm_xen_exit {
 #define KVM_EXIT_RISCV_CSR        36
 #define KVM_EXIT_NOTIFY           37
 #define KVM_EXIT_MEMORY_FAULT     38
+#define KVM_EXIT_VMGEXIT          50
 
 /* For KVM_EXIT_INTERNAL_ERROR */
 /* Emulate instruction failed. */
@@ -527,6 +528,11 @@ struct kvm_run {
 			__u64 gpa;
 			__u64 size;
 		} memory;
+		/* KVM_EXIT_VMGEXIT */
+		struct {
+			__u64 ghcb_msr; /* GHCB MSR contents */
+			__u64 ret; /* user -> kernel */
+		} vmgexit;
 		/* Fix the size of the union. */
 		char padding[256];
 	};
-- 
2.25.1