From: Xin Li <xin3.li@intel.com>

Save guest FRED RSP0 in vmx_prepare_switch_to_host() and restore it
in vmx_prepare_switch_to_guest() because MSR_IA32_FRED_RSP0 is passed
through to the guest, the guest value is volatile/unknown.

Note, host FRED RSP0 is restored in arch_exit_to_user_mode_prepare(),
regardless of whether it is modified in KVM.

Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
---

Changes since v2:
* KVM only needs to save/restore guest FRED RSP0 now as host FRED RSP0
  is restored in arch_exit_to_user_mode_prepare() (Sean Christopherson).

Changes since v1:
* Don't use guest_cpuid_has() in vmx_prepare_switch_to_{host,guest}(),
  which are called from IRQ-disabled context (Chao Gao).
* Reset msr_guest_fred_rsp0 in __vmx_vcpu_reset() (Chao Gao).
---
 arch/x86/kvm/vmx/vmx.c | 8 ++++++++
 arch/x86/kvm/vmx/vmx.h | 1 +
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c10c955722a3..c638492ebd59 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1348,6 +1348,9 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	}
 
 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
+
+	if (cpu_feature_enabled(X86_FEATURE_FRED) && guest_can_use(vcpu, X86_FEATURE_FRED))
+		wrmsrns(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
 #else
 	savesegment(fs, fs_sel);
 	savesegment(gs, gs_sel);
@@ -1392,6 +1395,11 @@ static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
 	invalidate_tss_limit();
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
+
+	if (cpu_feature_enabled(X86_FEATURE_FRED) && guest_can_use(&vmx->vcpu, X86_FEATURE_FRED)) {
+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
+		fred_sync_rsp0(vmx->msr_guest_fred_rsp0);
+	}
 #endif
 	load_fixmap_gdt(raw_smp_processor_id());
 	vmx->guest_state_loaded = false;
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index e7409f8f28b1..9ba960472c5f 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -277,6 +277,7 @@ struct vcpu_vmx {
 #ifdef CONFIG_X86_64
 	u64		      msr_host_kernel_gs_base;
 	u64		      msr_guest_kernel_gs_base;
+	u64		      msr_guest_fred_rsp0;
 #endif
 
 	u64		      spec_ctrl;
-- 
2.46.2