[PATCH v10 12/19] KVM: xen: allow vcpu_info to be mapped by fixed HVA

Paul Durrant
2023-12-04T15:01:24Z

If the guest does not explicitly set the GPA of vcpu_info structure in
memory then, for guests with 32 vCPUs or fewer, the vcpu_info embedded
in the shared_info page may be used. As described in a previous commit,
the shared_info page is an overlay at a fixed HVA within the VMM, so in
this case it also more optimal to activate the vcpu_info cache with a
fixed HVA to avoid unnecessary invalidation if the guest memory layout
is modified.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org

v8:
- Re-base.

v5:
- New in this version.
---
Documentation/virt/kvm/api.rst | 26 +++++++++++++++++++++-----
arch/x86/kvm/xen.c | 34 ++++++++++++++++++++++++++++------
include/uapi/linux/kvm.h | 3 +++
3 files changed, 52 insertions(+), 11 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 689264502a67..58927d5ce502 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5535,11 +5535,12 @@ KVM_XEN_ATTR_TYPE_SHARED_INFO
Sets the guest physical frame number at which the Xen shared_info
page resides. Note that although Xen places vcpu_info for the first
32 vCPUs in the shared_info page, KVM does not automatically do so
- and instead requires that KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO be used
- explicitly even when the vcpu_info for a given vCPU resides at the
- "default" location in the shared_info page. This is because KVM may
- not be aware of the Xen CPU id which is used as the index into the
- vcpu_info[] array, so may know the correct default location.
+ and instead requires that KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO or
+ KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA be used explicitly even when
+ the vcpu_info for a given vCPU resides at the "default" location
+ in the shared_info page. This is because KVM may not be aware of
+ the Xen CPU id which is used as the index into the vcpu_info[]
+ array, so may know the correct default location.

Note that the shared_info page may be constantly written to by KVM;
it contains the event channel bitmap used to deliver interrupts to
@@ -5661,6 +5662,21 @@ KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO
on dirty logging. Setting the gpa to KVM_XEN_INVALID_GPA will disable
the vcpu_info.

+KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA
+ If the KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA flag is also set in the
+ Xen capabilities, then this attribute may be used to set the
+ userspace address of the vcpu_info for a given vCPU. It should
+ only be used when the vcpu_info resides at the "default" location
+ in the shared_info page. In this case it is safe to assume the
+ userspace address will not change, because the shared_info page is
+ an overlay on guest memory and remains at a fixed host address
+ regardless of where it is mapped in guest physical address space
+ and hence unnecessary invalidation of an internal cache may be
+ avoided if the guest memory layout is modified.
+ If the vcpu_info does not reside at the "default" location then
+ it is not guaranteed to remain at the same host address and
+ hence the aforementioned cache invalidation is required.
+
KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO
Sets the guest physical address of an additional pvclock structure
for a given vCPU. This is typically used for guest vsyscall support.
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index d65b62f19440..c6f943362de5 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -778,20 +778,33 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)

switch (data->type) {
case KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO:
+ case KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA:
/* No compat necessary here. */
BUILD_BUG_ON(sizeof(struct vcpu_info) !=
sizeof(struct compat_vcpu_info));
BUILD_BUG_ON(offsetof(struct vcpu_info, time) !=
offsetof(struct compat_vcpu_info, time));

- if (data->u.gpa == KVM_XEN_INVALID_GPA) {
- kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
- r = 0;
- break;
+ if (data->type == KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO) {
+ if (data->u.gpa == KVM_XEN_INVALID_GPA) {
+ kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
+ r = 0;
+ break;
+ }
+
+ r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
+ data->u.gpa, sizeof(struct vcpu_info));
+ } else {
+ if (data->u.hva == 0) {
+ kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
+ r = 0;
+ break;
+ }
+
+ r = kvm_gpc_activate_hva(&vcpu->arch.xen.vcpu_info_cache,
+ data->u.hva, sizeof(struct vcpu_info));
}

- r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
- data->u.gpa, sizeof(struct vcpu_info));
if (!r)
kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);

@@ -1020,6 +1033,15 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
r = 0;
break;

+ case KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA:
+ if (vcpu->arch.xen.vcpu_info_cache.active &&
+ vcpu->arch.xen.vcpu_info_cache.gpa == KVM_XEN_INVALID_GPA)
+ data->u.hva = vcpu->arch.xen.vcpu_info_cache.uhva;
+ else
+ data->u.hva = 0;
+ r = 0;
+ break;
+
case KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO:
if (vcpu->arch.xen.vcpu_time_info_cache.active)
data->u.gpa = vcpu->arch.xen.vcpu_time_info_cache.gpa;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 32cf3432a507..9628a8018500 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1898,6 +1898,7 @@ struct kvm_xen_vcpu_attr {
union {
__u64 gpa;
#define KVM_XEN_INVALID_GPA ((__u64)-1)
+ __u64 hva;
__u64 pad[8];
struct {
__u64 state;
@@ -1928,6 +1929,8 @@ struct kvm_xen_vcpu_attr {
#define KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID 0x6
#define KVM_XEN_VCPU_ATTR_TYPE_TIMER 0x7
#define KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR 0x8
+/* Available with KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA */
+#define KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA 0x9

/* Secure Encrypted Virtualization command */
enum sev_cmd_id {
--
2.39.2