[PATCH v6 18/21] kvm: x86: Add support for getting/setting expanded xstate buffer

Paolo Bonzini
2022-01-07T18:56:11Z

From: Guang Zeng <guang.zeng@intel.com>

With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set
xstate data from/to KVM. This doesn't work when dynamic xfeatures
(e.g. AMX) are exposed to the guest as they require a larger buffer
size.

Introduce a new capability (KVM_CAP_XSAVE2). Userspace VMM gets the
required xstate buffer size via KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
KVM_SET_XSAVE is extended to work with both legacy and new capabilities
by doing properly-sized memdup_user() based on the guest fpu container.
KVM_GET_XSAVE is kept for backward-compatible reason. Instead,
KVM_GET_XSAVE2 is introduced under KVM_CAP_XSAVE2 as the preferred
interface for getting xstate buffer (4KB or larger size) from KVM
(Link: https://lkml.org/lkml/2021/12/15/510)

Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.

Signed-off-by: Guang Zeng <guang.zeng@intel.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-19-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
Documentation/virt/kvm/api.rst | 42 ++++++++++++++++++++++++++++--
arch/x86/include/uapi/asm/kvm.h | 16 +++++++++++-
arch/x86/kvm/cpuid.c | 2 +-
arch/x86/kvm/cpuid.h | 2 ++
arch/x86/kvm/x86.c | 45 ++++++++++++++++++++++++++++++++-
include/uapi/linux/kvm.h | 4 +++
6 files changed, 106 insertions(+), 5 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index f4ea5e41a4d0..d3791a14eb9a 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1569,6 +1569,7 @@ otherwise it will return EBUSY error.

struct kvm_xsave {
__u32 region[1024];
+ __u32 extra[0];
};

This ioctl would copy current vcpu's xsave struct to the userspace.
@@ -1577,7 +1578,7 @@ This ioctl would copy current vcpu's xsave struct to the userspace.
4.43 KVM_SET_XSAVE
------------------

-:Capability: KVM_CAP_XSAVE
+:Capability: KVM_CAP_XSAVE and KVM_CAP_XSAVE2
:Architectures: x86
:Type: vcpu ioctl
:Parameters: struct kvm_xsave (in)
@@ -1588,9 +1589,18 @@ This ioctl would copy current vcpu's xsave struct to the userspace.

struct kvm_xsave {
__u32 region[1024];
+ __u32 extra[0];
};

-This ioctl would copy userspace's xsave struct to the kernel.
+This ioctl would copy userspace's xsave struct to the kernel. It copies
+as many bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2),
+when invoked on the vm file descriptor. The size value returned by
+KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will always be at least 4096.
+Currently, it is only greater than 4096 if a dynamic feature has been
+enabled with ``arch_prctl()``, but this may change in the future.
+
+The offsets of the state save areas in struct kvm_xsave follow the
+contents of CPUID leaf 0xD on the host.

4.44 KVM_GET_XCRS
@@ -5535,6 +5545,34 @@ the trailing ``'\0'``, is indicated by ``name_size`` in the header.
The Stats Data block contains an array of 64-bit values in the same order
as the descriptors in Descriptors block.

+4.42 KVM_GET_XSAVE2
+------------------
+
+:Capability: KVM_CAP_XSAVE2
+:Architectures: x86
+:Type: vcpu ioctl
+:Parameters: struct kvm_xsave (out)
+:Returns: 0 on success, -1 on error
+
+
+::
+
+ struct kvm_xsave {
+ __u32 region[1024];
+ __u32 extra[0];
+ };
+
+This ioctl would copy current vcpu's xsave struct to the userspace. It
+copies as many bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2)
+when invoked on the vm file descriptor. The size value returned by
+KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will always be at least 4096.
+Currently, it is only greater than 4096 if a dynamic feature has been
+enabled with ``arch_prctl()``, but this may change in the future.
+
+The offsets of the state save areas in struct kvm_xsave follow the contents
+of CPUID leaf 0xD on the host.
+
+
5. The kvm_run structure
========================

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 5a776a08f78c..2da3316bb559 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -373,9 +373,23 @@ struct kvm_debugregs {
__u64 reserved[9];
};

-/* for KVM_CAP_XSAVE */
+/* for KVM_CAP_XSAVE and KVM_CAP_XSAVE2 */
struct kvm_xsave {
+ /*
+ * KVM_GET_XSAVE2 and KVM_SET_XSAVE write and read as many bytes
+ * as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2)
+ * respectively, when invoked on the vm file descriptor.
+ *
+ * The size value returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2)
+ * will always be at least 4096. Currently, it is only greater
+ * than 4096 if a dynamic feature has been enabled with
+ * ``arch_prctl()``, but this may change in the future.
+ *
+ * The offsets of the state save areas in struct kvm_xsave follow
+ * the contents of CPUID leaf 0xD on the host.
+ */
__u32 region[1024];
+ __u32 extra[0];
};

#define KVM_MAX_XCRS 16
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index ba4c3d5d2386..c55e57b30e81 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -32,7 +32,7 @@
u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
EXPORT_SYMBOL_GPL(kvm_cpu_caps);

-static u32 xstate_required_size(u64 xstate_bv, bool compacted)
+u32 xstate_required_size(u64 xstate_bv, bool compacted)
{
int feature_bit = 0;
u32 ret = XSAVE_HDR_SIZE + XSAVE_HDR_OFFSET;
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index c99edfff7f82..8a770b481d9d 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -30,6 +30,8 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
u32 *ecx, u32 *edx, bool exact_only);

+u32 xstate_required_size(u64 xstate_bv, bool compacted);
+
int cpuid_query_maxphyaddr(struct kvm_vcpu *vcpu);
u64 kvm_vcpu_reserved_gpa_bits_raw(struct kvm_vcpu *vcpu);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 993eee6451ea..bde18ca657db 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4314,6 +4314,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
else
r = 0;
break;
+ case KVM_CAP_XSAVE2: {
+ u64 guest_perm = xstate_get_guest_group_perm();
+
+ r = xstate_required_size(supported_xcr0 & guest_perm, false);
+ if (r < sizeof(struct kvm_xsave))
+ r = sizeof(struct kvm_xsave);
+ break;
+ }
default:
break;
}
@@ -4917,6 +4925,16 @@ static void kvm_vcpu_ioctl_x86_get_xsave(struct kvm_vcpu *vcpu,
vcpu->arch.pkru);
}

+static void kvm_vcpu_ioctl_x86_get_xsave2(struct kvm_vcpu *vcpu,
+ u8 *state, unsigned int size)
+{
+ if (fpstate_is_confidential(&vcpu->arch.guest_fpu))
+ return;
+
+ fpu_copy_guest_fpstate_to_uabi(&vcpu->arch.guest_fpu,
+ state, size, vcpu->arch.pkru);
+}
+
static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
struct kvm_xsave *guest_xsave)
{
@@ -5370,6 +5388,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
break;
}
case KVM_GET_XSAVE: {
+ r = -EINVAL;
+ if (vcpu->arch.guest_fpu.uabi_size > sizeof(struct kvm_xsave))
+ break;
+
u.xsave = kzalloc(sizeof(struct kvm_xsave), GFP_KERNEL_ACCOUNT);
r = -ENOMEM;
if (!u.xsave)
@@ -5384,7 +5406,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
break;
}
case KVM_SET_XSAVE: {
- u.xsave = memdup_user(argp, sizeof(*u.xsave));
+ int size = vcpu->arch.guest_fpu.uabi_size;
+
+ u.xsave = memdup_user(argp, size);
if (IS_ERR(u.xsave)) {
r = PTR_ERR(u.xsave);
goto out_nofree;
@@ -5393,6 +5417,25 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
r = kvm_vcpu_ioctl_x86_set_xsave(vcpu, u.xsave);
break;
}
+
+ case KVM_GET_XSAVE2: {
+ int size = vcpu->arch.guest_fpu.uabi_size;
+
+ u.xsave = kzalloc(size, GFP_KERNEL_ACCOUNT);
+ r = -ENOMEM;
+ if (!u.xsave)
+ break;
+
+ kvm_vcpu_ioctl_x86_get_xsave2(vcpu, u.buffer, size);
+
+ r = -EFAULT;
+ if (copy_to_user(argp, u.xsave, size))
+ break;
+
+ r = 0;
+ break;
+ }
+
case KVM_GET_XCRS: {
u.xcrs = kzalloc(sizeof(struct kvm_xcrs), GFP_KERNEL_ACCOUNT);
r = -ENOMEM;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index fbfd70d965c6..9563d294f181 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1132,6 +1132,7 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_ARM_MTE 205
#define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
#define KVM_CAP_VM_GPA_BITS 207
+#define KVM_CAP_XSAVE2 208

#ifdef KVM_CAP_IRQ_ROUTING

@@ -1622,6 +1623,9 @@ struct kvm_enc_region {
#define KVM_S390_NORMAL_RESET _IO(KVMIO, 0xc3)
#define KVM_S390_CLEAR_RESET _IO(KVMIO, 0xc4)

+/* Available with KVM_CAP_XSAVE2 */
+#define KVM_GET_XSAVE2 _IOR(KVMIO, 0xcf, struct kvm_xsave)
+
struct kvm_s390_pv_sec_parm {
__u64 origin;
__u64 length;
--
2.31.1