[PATCH RFC v9 35/51] KVM: SVM: Add KVM_EXIT_VMGEXIT

Michael Roth
2023-06-12T04:46:15Z

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