[PATCH RFC v5 24/53] KVM: SEV: Make 'uaddr' parameter optional for KVM_SEV_SNP_LAUNCH_UPDATE

Ackerley Tng via B4 Relay
2026-04-28T23:25:19Z

From: Michael Roth <michael.roth@amd.com>

For vm_memory_attributes=1, in-place conversion/population is not
supported, so the initial contents necessarily must need to come
from a separate src address, which is enforced by the current
implementation. However, for vm_memory_attributes=0, it is possible for
guest memory to be initialized directly from userspace by mmap()'ing the
guest_memfd and writing to it while the corresponding GPA ranges are in
a 'shared' state before converting them to the 'private' state expected
by KVM_SEV_SNP_LAUNCH_UPDATE.

Update the handling/documentation for KVM_SEV_SNP_LAUNCH_UPDATE to allow
for 'uaddr' to be set to NULL when vm_memory_attributes=0, which
SNP_LAUNCH_UPDATE will then use to determine when it should/shouldn't
copy in data from a separate memory location. Continue to enforce
non-NULL for the original vm_memory_attributes=1 case.

Signed-off-by: Michael Roth <michael.roth@amd.com>
[Added src_page check in error handling path when the firmware command fails]
[Dropped ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES]
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
Documentation/virt/kvm/x86/amd-memory-encryption.rst | 19 +++++++++++++++----
arch/x86/kvm/svm/sev.c | 18 +++++++++++++-----
virt/kvm/kvm_main.c | 1 +
3 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/Documentation/virt/kvm/x86/amd-memory-encryption.rst b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
index b2395dd4769de..3b9f36a55a95b 100644
--- a/Documentation/virt/kvm/x86/amd-memory-encryption.rst
+++ b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
@@ -503,7 +503,12 @@ secrets.

It is required that the GPA ranges initialized by this command have had the
KVM_MEMORY_ATTRIBUTE_PRIVATE attribute set in advance. See the documentation
-for KVM_SET_MEMORY_ATTRIBUTES for more details on this aspect.
+for KVM_SET_MEMORY_ATTRIBUTES/KVM_SET_MEMORY_ATTRIBUTES2 for more details on
+this aspect. If running with kvm.vm_memory_attributes=0 (to allow for
+guest_memfd to handle memory attributes and allow for in-place conversion of
+pages between shared/private), the 'PRESERVED' flag/content mode (which is
+only available via the KVM_SET_MEMORY_ATTRIBUTES2 interface) must be used
+when setting the range to private prior to issuing this ioctl.

Upon success, this command is not guaranteed to have processed the entire
range requested. Instead, the ``gfn_start``, ``uaddr``, and ``len`` fields of
@@ -511,9 +516,15 @@ range requested. Instead, the ``gfn_start``, ``uaddr``, and ``len`` fields of
remaining range that has yet to be processed. The caller should continue
calling this command until those fields indicate the entire range has been
processed, e.g. ``len`` is 0, ``gfn_start`` is equal to the last GFN in the
-range plus 1, and ``uaddr`` is the last byte of the userspace-provided source
-buffer address plus 1. In the case where ``type`` is KVM_SEV_SNP_PAGE_TYPE_ZERO,
-``uaddr`` will be ignored completely.
+range plus 1, and ``uaddr`` (if specified) is the last byte of the
+userspace-provided source buffer address plus 1.
+
+In the case where ``type`` is KVM_SEV_SNP_PAGE_TYPE_ZERO, ``uaddr`` will be
+ignored completely. Otherwise, ``uaddr`` is required if
+kvm.vm_memory_attributes=0 and optional if kvm.vm_memory_attributes=1, since
+in the latter case guest memory can be initialized directly from userspace
+prior to converting it to private and passing the GPA range on to this
+interface.

Parameters (in): struct kvm_sev_snp_launch_update

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index c2126b3c30724..bf10d24907a00 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2343,7 +2343,15 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
int level;
int ret;

- if (WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO && !src_page))
+ /*
+ * For vm_memory_attributes=1, in-place conversion/population is not
+ * supported, so the initial contents necessarily need to come from a
+ * separate src address. For vm_memory_attributes=0, this isn't
+ * necessarily the case, since the pages may have been populated
+ * directly from userspace before calling KVM_SEV_SNP_LAUNCH_UPDATE.
+ */
+ if (vm_memory_attributes &&
+ sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO && !src_page)
return -EINVAL;

ret = snp_lookup_rmpentry((u64)pfn, &assigned, &level);
@@ -2390,7 +2398,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
*/
if (ret && !snp_page_reclaim(kvm, pfn) &&
sev_populate_args->type == KVM_SEV_SNP_PAGE_TYPE_CPUID &&
- sev_populate_args->fw_error == SEV_RET_INVALID_PARAM) {
+ sev_populate_args->fw_error == SEV_RET_INVALID_PARAM && src_page) {
void *src_vaddr = kmap_local_page(src_page);
void *dst_vaddr = kmap_local_pfn(pfn);

@@ -2422,8 +2430,8 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
if (copy_from_user(&params, u64_to_user_ptr(argp->data), sizeof(params)))
return -EFAULT;

- pr_debug("%s: GFN start 0x%llx length 0x%llx type %d flags %d\n", __func__,
- params.gfn_start, params.len, params.type, params.flags);
+ pr_debug("%s: GFN start 0x%llx length 0x%llx type %d flags %d src %llx\n", __func__,
+ params.gfn_start, params.len, params.type, params.flags, params.uaddr);

if (!params.len || !PAGE_ALIGNED(params.len) || params.flags ||
(params.type != KVM_SEV_SNP_PAGE_TYPE_NORMAL &&
@@ -2479,7 +2487,7 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)

params.gfn_start += count;
params.len -= count * PAGE_SIZE;
- if (params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO)
+ if (src && params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO)
params.uaddr += count * PAGE_SIZE;

if (copy_to_user(u64_to_user_ptr(argp->data), &params, sizeof(params)))
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ba195bb239aaa..3bf212fd99193 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -105,6 +105,7 @@ module_param(allow_unsafe_mappings, bool, 0444);
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
bool vm_memory_attributes = true;
module_param(vm_memory_attributes, bool, 0444);
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(vm_memory_attributes);
#endif
DEFINE_STATIC_CALL_RET0(__kvm_get_memory_attributes, kvm_get_memory_attributes_t);
EXPORT_SYMBOL_FOR_KVM_INTERNAL(STATIC_CALL_KEY(__kvm_get_memory_attributes));

--
2.54.0.545.g6539524ca2-goog