[PATCH v3 07/19] accel/kvm: Add CGS flag to control in-place conversion support

[Michael Roth <michael.roth@amd.com>]

For confidential guests, guest_memfd is currently used only for private
guest memory, and normal guest memory comes from the configured memory
backend just as it does for a non-confidential guest. It is now possible
to use the same physical memory to back a particular GPA regardless of
whether it is in a shared or private state. This avoids the need to
rely on discarding memory between shared/private conversions (to avoid
doubled memory usage), and is intended to be the primary mode of using
guest_memfd for confidential guests moving forward, and future features
like hugepage support will likely require it.

Add an internal flag to enable this support. Since
ConfidentialGuestSupport is already used to track some
guest_memfd-related functionality (e.g.  whether it is required for the
configured machine), similarly introduce this flag as a member of
the ConfidentialGuestSupport object.

Also add the KVM-specific checks to enable this support, but leave the
option disabled so that required changes can first be implemented for
CGS variants that intend to make use of KVM's in-place conversion
support.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 accel/kvm/kvm-all.c                         | 18 ++++++++++++++++++
 hw/core/machine.c                           |  5 +++++
 include/hw/core/boards.h                    |  1 +
 include/system/confidential-guest-support.h | 12 ++++++++++++
 4 files changed, 36 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index f1deb458ec..862539aed4 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -3074,6 +3074,24 @@ static int kvm_init(AccelState *as, MachineState *ms)
     kvm_guest_memfd_flags_supported =
         kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_FLAGS);
 
+    if (machine_require_guest_memfd_convert_in_place(ms)) {
+        uint64_t guest_memfd_supported_memory_attributes;
+
+        guest_memfd_supported_memory_attributes =
+            kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES);
+
+        if (!(guest_memfd_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
+            ret = -EINVAL;
+            error_report("In-place conversion is only supported if private "
+                         "memory attributes can be set via guest_memfd. "
+                         "Please ensure the 'gmem_in_place_conversion' KVM "
+                         "module parameter is set to 1.");
+            goto err;
+        }
+
+        kvm_supported_memory_attributes = guest_memfd_supported_memory_attributes;
+    }
+
     if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
         s->kernel_irqchip_split = mc->default_kernel_irqchip_split ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
     }
diff --git a/hw/core/machine.c b/hw/core/machine.c
index d9ac4b419a..e9cc6c2420 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -1346,6 +1346,11 @@ bool machine_require_guest_memfd_private(MachineState *machine)
     return machine->cgs && machine->cgs->require_guest_memfd;
 }
 
+bool machine_require_guest_memfd_convert_in_place(MachineState *machine)
+{
+    return machine->cgs && machine->cgs->convert_in_place;
+}
+
 static char *cpu_slot_to_string(const CPUArchId *cpu)
 {
     GString *s = g_string_new(NULL);
diff --git a/include/hw/core/boards.h b/include/hw/core/boards.h
index d0e4ad9820..9ea124b224 100644
--- a/include/hw/core/boards.h
+++ b/include/hw/core/boards.h
@@ -44,6 +44,7 @@ int machine_phandle_start(MachineState *machine);
 bool machine_dump_guest_core(MachineState *machine);
 bool machine_mem_merge(MachineState *machine);
 bool machine_require_guest_memfd_private(MachineState *machine);
+bool machine_require_guest_memfd_convert_in_place(MachineState *machine);
 HotpluggableCPUList *machine_query_hotpluggable_cpus(MachineState *machine);
 void machine_set_cpu_numa_node(MachineState *machine,
                                const CpuInstanceProperties *props,
diff --git a/include/system/confidential-guest-support.h b/include/system/confidential-guest-support.h
index 5dca717308..2c9a9ff217 100644
--- a/include/system/confidential-guest-support.h
+++ b/include/system/confidential-guest-support.h
@@ -92,6 +92,18 @@ struct ConfidentialGuestSupport {
      * so 'ready' is not set, we'll abort.
      */
     bool ready;
+
+    /*
+     * CGS implementations will use this to indicate whether or not
+     * to enable in-place conversion for guest-memfd.
+     *
+     * If set, the machine re-uses physical pages when converting
+     * between shared/private (as opposed to using different
+     * physical pages depending on the access type, which is the
+     * default mode when 'require_guest_memfd' is set without
+     * additionally setting this flag).
+     */
+    bool convert_in_place;
 };
 
 typedef struct ConfidentialGuestSupportClass {
-- 
2.43.0