Re: [RFC 03/33] KVM: x86: hyper-v: Introduce XMM output support

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On 08.11.23 13:11, Vitaly Kuznetsov wrote:
Alexander Graf <graf@xxxxxxxxxx> writes:

On 08.11.23 12:17, Nicolas Saenz Julienne wrote:
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@xxxxxxxxxx>
---
   arch/x86/include/asm/hyperv-tlfs.h |  2 +-
   arch/x86/kvm/hyperv.c              | 33 +++++++++++++++++++++++++++++-
   include/uapi/linux/kvm.h           |  6 ++++++
   3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 2ff26f53cd62..af594aa65307 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 238afd7335e4..e1bc861ab3b0 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1815,6 +1815,7 @@ struct kvm_hv_hcall {
      u16 rep_idx;
      bool fast;
      bool rep;
+    bool xmm_dirty;
      sse128_t xmm[HV_HYPERCALL_MAX_XMM_REGISTERS];

      /*
@@ -2346,9 +2347,33 @@ 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 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 (kvm_hv_is_xmm_output_hcall(code) && hv_result_success(result) && fast)
+            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)
@@ -2623,6 +2648,9 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
              break;
      }

+    if ((ret & HV_HYPERCALL_RESULT_MASK) == HV_STATUS_SUCCESS && hc.xmm_dirty)
+            kvm_hv_write_xmm((struct kvm_hyperv_xmm_reg*)hc.xmm);
+
   hypercall_complete:
      return kvm_hv_hypercall_complete(vcpu, ret);

@@ -2632,6 +2660,8 @@ 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;
   }
@@ -2780,6 +2810,7 @@ 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;

Shouldn't this be guarded by an ENABLE_CAP to make sure old user space
that doesn't know about xmm outputs is still able to run with newer kernels?

No, we don't do CAPs for new Hyper-V features anymore since we have
KVM_GET_SUPPORTED_HV_CPUID. Userspace is not supposed to simply copy
its output into guest visible CPUIDs, it must only enable features it
knows. Even 'hv_passthrough' option in QEMU doesn't pass unknown
features through.


Ah, nice :). That simplifies things.


Alex




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