[PATCH 01/18] KVM: x86: hyper-v: Introduce XMM output support

Nicolas Saenz Julienne
2024-06-09T15:51:04Z

Prepare infrastructure to be able to return data through the XMM
registers when Hyper-V hypercalls are issues in fast mode. The XMM
registers are exposed to user-space through KVM_EXIT_HYPERV_HCALL and
restored on successful hypercall completion.

Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>

---

There was some discussion in the RFC about whether growing 'struct
kvm_hyperv_exit' is ABI breakage. IMO it isn't:
- There is padding in 'struct kvm_run' that ensures that a bigger
'struct kvm_hyperv_exit' doesn't alter the offsets within that struct.
- Adding a new field at the bottom of the 'hcall' field within the
'struct kvm_hyperv_exit' should be fine as well, as it doesn't alter
the offsets within that struct either.
- Ultimately, previous updates to 'struct kvm_hyperv_exit's hint that
its size isn't part of the uABI. It already grew when syndbg was
introduced.

Documentation/virt/kvm/api.rst | 19 ++++++++++
arch/x86/include/asm/hyperv-tlfs.h | 2 +-
arch/x86/kvm/hyperv.c | 56 +++++++++++++++++++++++++++++-
include/uapi/linux/kvm.h | 6 ++++
4 files changed, 81 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index a71d91978d9ef..17893b330b76f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8893,3 +8893,22 @@ Ordering of KVM_GET_*/KVM_SET_* ioctls
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

TBD
+
+10. Hyper-V CPUIDs
+==================
+
+This section only applies to x86.
+
+New Hyper-V feature support is no longer being tracked through KVM
+capabilities. Userspace can check if a particular version of KVM supports a
+feature using KMV_GET_SUPPORTED_HV_CPUID. This section documents how Hyper-V
+CPUIDs map to KVM functionality.
+
+10.1 HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE
+------------------------------------------
+
+:Location: CPUID.40000003H:EDX[bit 15]
+
+This CPUID indicates that KVM supports retuning data to the guest in response
+to a hypercall using the XMM registers. It also extends ``struct
+kvm_hyperv_exit`` to allow passing the XMM data from userspace.
diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 3787d26810c1c..6a18c9f77d5fe 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h
@@ -49,7 +49,7 @@
/* Support for physical CPU dynamic partitioning events is available*/
#define HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE BIT(3)
/*
- * Support for passing hypercall input parameter block via XMM
+ * Support for passing hypercall input and output parameter block via XMM
* registers is available
*/
#define HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE BIT(4)
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 8a47f8541eab7..42f44546fe79c 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1865,6 +1865,7 @@ struct kvm_hv_hcall {
u16 rep_idx;
bool fast;
bool rep;
+ bool xmm_dirty;
sse128_t xmm[HV_HYPERCALL_MAX_XMM_REGISTERS];

/*
@@ -2396,9 +2397,49 @@ static int kvm_hv_hypercall_complete(struct kvm_vcpu *vcpu, u64 result)
return ret;
}

+static void kvm_hv_write_xmm(struct kvm_hyperv_xmm_reg *xmm)
+{
+ int reg;
+
+ kvm_fpu_get();
+ for (reg = 0; reg < HV_HYPERCALL_MAX_XMM_REGISTERS; reg++) {
+ const sse128_t data = sse128(xmm[reg].low, xmm[reg].high);
+ _kvm_write_sse_reg(reg, &data);
+ }
+ kvm_fpu_put();
+}
+
+static bool kvm_hv_is_xmm_output_hcall(u16 code)
+{
+ return false;
+}
+
+static bool kvm_hv_xmm_output_allowed(struct kvm_vcpu *vcpu)
+{
+ struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
+
+ return !hv_vcpu->enforce_cpuid ||
+ hv_vcpu->cpuid_cache.features_edx &
+ HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE;
+}
+
static int kvm_hv_hypercall_complete_userspace(struct kvm_vcpu *vcpu)
{
- return kvm_hv_hypercall_complete(vcpu, vcpu->run->hyperv.u.hcall.result);
+ bool fast = !!(vcpu->run->hyperv.u.hcall.input & HV_HYPERCALL_FAST_BIT);
+ u16 code = vcpu->run->hyperv.u.hcall.input & 0xffff;
+ u64 result = vcpu->run->hyperv.u.hcall.result;
+
+ if (hv_result_success(result) && fast &&
+ kvm_hv_is_xmm_output_hcall(code)) {
+ if (unlikely(!kvm_hv_xmm_output_allowed(vcpu))) {
+ kvm_queue_exception(vcpu, UD_VECTOR);
+ return 1;
+ }
+
+ kvm_hv_write_xmm(vcpu->run->hyperv.u.hcall.xmm);
+ }
+
+ return kvm_hv_hypercall_complete(vcpu, result);
}

static u16 kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
@@ -2553,6 +2594,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
hc.rep_cnt = (hc.param >> HV_HYPERCALL_REP_COMP_OFFSET) & 0xfff;
hc.rep_idx = (hc.param >> HV_HYPERCALL_REP_START_OFFSET) & 0xfff;
hc.rep = !!(hc.rep_cnt || hc.rep_idx);
+ hc.xmm_dirty = false;

trace_kvm_hv_hypercall(hc.code, hc.fast, hc.var_cnt, hc.rep_cnt,
hc.rep_idx, hc.ingpa, hc.outgpa);
@@ -2673,6 +2715,15 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
break;
}

+ if (hv_result_success(ret) && hc.xmm_dirty) {
+ if (unlikely(!kvm_hv_xmm_output_allowed(vcpu))) {
+ kvm_queue_exception(vcpu, UD_VECTOR);
+ return 1;
+ }
+
+ kvm_hv_write_xmm((struct kvm_hyperv_xmm_reg *)hc.xmm);
+ }
+
hypercall_complete:
return kvm_hv_hypercall_complete(vcpu, ret);

@@ -2682,6 +2733,8 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
vcpu->run->hyperv.u.hcall.input = hc.param;
vcpu->run->hyperv.u.hcall.params[0] = hc.ingpa;
vcpu->run->hyperv.u.hcall.params[1] = hc.outgpa;
+ if (hc.fast)
+ memcpy(vcpu->run->hyperv.u.hcall.xmm, hc.xmm, sizeof(hc.xmm));
vcpu->arch.complete_userspace_io = kvm_hv_hypercall_complete_userspace;
return 0;
}
@@ -2830,6 +2883,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
ent->ebx |= HV_ENABLE_EXTENDED_HYPERCALLS;

ent->edx |= HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE;
+ ent->edx |= HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE;
ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
ent->edx |= HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d03842abae578..fbdee8d754595 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -90,6 +90,11 @@ struct kvm_pit_config {

#define KVM_PIT_SPEAKER_DUMMY 1

+struct kvm_hyperv_xmm_reg {
+ __u64 low;
+ __u64 high;
+};
+
struct kvm_hyperv_exit {
#define KVM_EXIT_HYPERV_SYNIC 1
#define KVM_EXIT_HYPERV_HCALL 2
@@ -108,6 +113,7 @@ struct kvm_hyperv_exit {
__u64 input;
__u64 result;
__u64 params[2];
+ struct kvm_hyperv_xmm_reg xmm[6];
} hcall;
struct {
__u32 msr;
--
2.40.1