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Message-ID: <diqz4irqg9qy.fsf@google.com>
Date: Wed, 22 Oct 2025 09:51:49 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Steven Price <steven.price@....com>, cgroups@...r.kernel.org, kvm@...r.kernel.org,
linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2
Steven Price <steven.price@....com> writes:
> On 17/10/2025 21:11, Ackerley Tng wrote:
>>
>> [...snip...]
>>
>> @@ -5366,15 +5375,35 @@ static long kvm_vm_ioctl(struct file *filp,
>> }
>> #endif /* CONFIG_HAVE_KVM_IRQ_ROUTING */
>> #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
>> + case KVM_SET_MEMORY_ATTRIBUTES2:
>> case KVM_SET_MEMORY_ATTRIBUTES: {
>> - struct kvm_memory_attributes attrs;
>> + struct kvm_memory_attributes2 attrs;
>> + unsigned long size;
>> +
>> + if (ioctl == KVM_SET_MEMORY_ATTRIBUTES) {
>> + /*
>> + * Fields beyond struct kvm_userspace_memory_region shouldn't be
>> + * accessed, but avoid leaking kernel memory in case of a bug.
>> + */
>> + memset(&mem, 0, sizeof(mem));
>
> s/mem/attrs/g
>
>> + size = sizeof(struct kvm_set_memory_attributes);
>> + } else {
>> + size = sizeof(struct kvm_set_memory_attributes2);
>
> s/kvm_set_memory_attributes/kvm_memory_attributes/ (on both sizeof lines
> above and in the SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD macro).
>
>> + }
>> +
>> + /* Ensure the common parts of the two structs are identical. */
>> + SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(slot);
>> + SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(flags);
>> + SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(guest_phys_addr);
>> + SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(memory_size);
>> + SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(userspace_addr);
>
> The fields are:
> * address
> * size
> * attributes
> * flags
>
> The list you've got appears to match struct kvm_userspace_memory_region
> - copy/paste error?
>
Yes I did copy/paste this from KVM_SET_USER_MEMORY_REGION2.
Thanks for catching this! I missed out build-testing this with
CONFIG_KVM_VM_MEMORY_ATTRIBUTES.
I've done that and here's a replacement patch.
> Thanks,
> Steve
>
>>
>> [...snip...]
>>
>From 31283972574bde2ffa1960d30c80286f8467c594 Mon Sep 17 00:00:00 2001
From: Ackerley Tng <ackerleytng@...gle.com>
Date: Thu, 16 Oct 2025 11:48:01 -0700
Subject: [PATCH] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2
Introduce a "version 2" of KVM_SET_MEMORY_ATTRIBUTES to support returning
information back to userspace.
This new ioctl and structure will, in a later patch, be shared as a
guest_memfd ioctl, where the padding in the new kvm_memory_attributes2
structure will be for writing the response from the guest_memfd ioctl to
userspace.
A new ioctl is necessary for these reasons:
1. KVM_SET_MEMORY_ATTRIBUTES is currently a write-only ioctl and does not
allow userspace to read fields. There's nothing in code (yet?) that
validates this, but using _IOWR for consistency would be prudent.
2. KVM_SET_MEMORY_ATTRIBUTES, when used as a guest_memfd ioctl, will need
an additional field to provide userspace with more error details.
Alternatively, a completely new ioctl could be defined, unrelated to
KVM_SET_MEMORY_ATTRIBUTES, but using the same ioctl number and struct for
the vm and guest_memfd ioctls streamlines the interface for userspace. In
addition, any memory attributes, implemented on the vm or guest_memfd
ioctl, can be easily shared with the other.
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Change-Id: I50cd506d9a28bf68a90e659015603de579569bc1
Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
---
Documentation/virt/kvm/api.rst | 32 ++++++++++++++++++++++++++++++++
include/uapi/linux/kvm.h | 12 ++++++++++++
virt/kvm/kvm_main.c | 34 +++++++++++++++++++++++++++++++---
3 files changed, 75 insertions(+), 3 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 754b662a453c3..a812769d79bf6 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6355,6 +6355,8 @@ S390:
Returns -EINVAL if the VM has the KVM_VM_S390_UCONTROL flag set.
Returns -EINVAL if called on a protected VM.
+.. _KVM_SET_MEMORY_ATTRIBUTES:
+
4.141 KVM_SET_MEMORY_ATTRIBUTES
-------------------------------
@@ -6512,6 +6514,36 @@ the capability to be present.
`flags` must currently be zero.
+4.144 KVM_SET_MEMORY_ATTRIBUTES2
+---------------------------------
+
+:Capability: KVM_CAP_MEMORY_ATTRIBUTES2
+:Architectures: x86
+:Type: vm ioctl
+:Parameters: struct kvm_memory_attributes2 (in/out)
+:Returns: 0 on success, <0 on error
+
+KVM_SET_MEMORY_ATTRIBUTES2 is an extension to
+KVM_SET_MEMORY_ATTRIBUTES that supports returning (writing) values to
+userspace. The original (pre-extension) fields are shared with
+KVM_SET_MEMORY_ATTRIBUTES identically.
+
+Attribute values are shared with KVM_SET_MEMORY_ATTRIBUTES.
+
+::
+
+ struct kvm_memory_attributes2 {
+ __u64 address;
+ __u64 size;
+ __u64 attributes;
+ __u64 flags;
+ __u64 reserved[4];
+ };
+
+ #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
+
+See also: :ref: `KVM_SET_MEMORY_ATTRIBUTES`.
+
.. _kvm_run:
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 52f6000ab0208..c300e38c7c9cd 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -963,6 +963,7 @@ struct kvm_enable_cap {
#define KVM_CAP_RISCV_MP_STATE_RESET 242
#define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243
#define KVM_CAP_GUEST_MEMFD_FLAGS 244
+#define KVM_CAP_MEMORY_ATTRIBUTES2 245
struct kvm_irq_routing_irqchip {
__u32 irqchip;
@@ -1617,4 +1618,15 @@ struct kvm_pre_fault_memory {
__u64 padding[5];
};
+/* Available with KVM_CAP_MEMORY_ATTRIBUTES2 */
+#define KVM_SET_MEMORY_ATTRIBUTES2 _IOWR(KVMIO, 0xd6, struct kvm_memory_attributes2)
+
+struct kvm_memory_attributes2 {
+ __u64 address;
+ __u64 size;
+ __u64 attributes;
+ __u64 flags;
+ __u64 reserved[4];
+};
+
#endif /* __LINUX_KVM_H */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 35166754a22b4..95aa51b334a70 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2621,7 +2621,7 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end,
return r;
}
static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
- struct kvm_memory_attributes *attrs)
+ struct kvm_memory_attributes2 *attrs)
{
gfn_t start, end;
@@ -4959,6 +4959,7 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
case KVM_CAP_DEVICE_CTRL:
return 1;
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
+ case KVM_CAP_MEMORY_ATTRIBUTES2:
case KVM_CAP_MEMORY_ATTRIBUTES:
if (!vm_memory_attributes)
return 0;
@@ -5184,6 +5185,14 @@ do { \
sizeof_field(struct kvm_userspace_memory_region2, field)); \
} while (0)
+#define SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(field) \
+do { \
+ BUILD_BUG_ON(offsetof(struct kvm_memory_attributes, field) != \
+ offsetof(struct kvm_memory_attributes2, field)); \
+ BUILD_BUG_ON(sizeof_field(struct kvm_memory_attributes, field) != \
+ sizeof_field(struct kvm_memory_attributes2, field)); \
+} while (0)
+
static long kvm_vm_ioctl(struct file *filp,
unsigned int ioctl, unsigned long arg)
{
@@ -5366,15 +5375,34 @@ static long kvm_vm_ioctl(struct file *filp,
}
#endif /* CONFIG_HAVE_KVM_IRQ_ROUTING */
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
+ case KVM_SET_MEMORY_ATTRIBUTES2:
case KVM_SET_MEMORY_ATTRIBUTES: {
- struct kvm_memory_attributes attrs;
+ struct kvm_memory_attributes2 attrs;
+ unsigned long size;
+
+ if (ioctl == KVM_SET_MEMORY_ATTRIBUTES) {
+ /*
+ * Fields beyond struct kvm_userspace_memory_region shouldn't be
+ * accessed, but avoid leaking kernel memory in case of a bug.
+ */
+ memset(&attrs, 0, sizeof(attrs));
+ size = sizeof(struct kvm_memory_attributes);
+ } else {
+ size = sizeof(struct kvm_memory_attributes2);
+ }
+
+ /* Ensure the common parts of the two structs are identical. */
+ SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(address);
+ SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(size);
+ SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(attributes);
+ SANITY_CHECK_MEMORY_ATTRIBUTES_FIELD(flags);
r = -ENOTTY;
if (!vm_memory_attributes)
goto out;
r = -EFAULT;
- if (copy_from_user(&attrs, argp, sizeof(attrs)))
+ if (copy_from_user(&attrs, argp, size))
goto out;
r = kvm_vm_ioctl_set_mem_attributes(kvm, &attrs);
--
2.51.0.915.g61a8936c21-goog
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