Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST_SSP

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On 5/23/2023 4:57 PM, Binbin Wu wrote:


On 5/11/2023 12:08 PM, Yang Weijiang wrote:
Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the VMM
can read/write the guest's SSP, e.g. to migrate CET state.  Use a synthetic
MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the same
consistency checks as the PL*_SSP MSRs, i.e. can share code.

Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
  arch/x86/include/uapi/asm/kvm_para.h |  1 +
  arch/x86/kvm/vmx/vmx.c               | 15 ++++++++++++---
  arch/x86/kvm/x86.c                   |  4 ++++
  3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 6e64b27b2c1e..7af465e4e0bd 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -58,6 +58,7 @@
  #define MSR_KVM_ASYNC_PF_INT    0x4b564d06
  #define MSR_KVM_ASYNC_PF_ACK    0x4b564d07
  #define MSR_KVM_MIGRATION_CONTROL    0x4b564d08
+#define MSR_KVM_GUEST_SSP    0x4b564d09
    struct kvm_steal_time {
      __u64 steal;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0ccaa467d7d3..72149156bbd3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2095,9 +2095,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
          break;
      case MSR_IA32_U_CET:
      case MSR_IA32_PL3_SSP:
+    case MSR_KVM_GUEST_SSP:
          if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
              return 1;
-        kvm_get_xsave_msr(msr_info);
+        if (msr_info->index == MSR_KVM_GUEST_SSP)
+            msr_info->data = vmcs_readl(GUEST_SSP);
According to the change of the kvm_cet_is_msr_accessible() below,
kvm_cet_is_msr_accessible() will return false for MSR_KVM_GUEST_SSP, then this code is unreachable?

No, when the access is initiated from host side, kvm_cet_is_msr_accessible() return true for MSR_KVM_GUEST_SSP.

So the code is reachable:

if (msr->host_initiated)

    return true;


[...]



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