[PATCH v1 09/23] KVM: VMX: Switch FRED RSP0 between host and guest

[Xin Li <xin3.li@intel.com>]

Switch MSR_IA32_FRED_RSP0 between host and guest in
vmx_prepare_switch_to_{host,guest}().

MSR_IA32_FRED_RSP0 is used during ring 3 event delivery only, thus
KVM, running on ring 0, can run safely with guest FRED RSP0, i.e.,
no need to switch between host/guest FRED RSP0 during VM entry and
exit.

KVM should switch to host FRED RSP0 before returning to user level,
and switch to guest FRED RSP0 before entering guest mode.

Tested-by: Shan Kang <shan.kang@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---
 arch/x86/kvm/vmx/vmx.c | 17 +++++++++++++++++
 arch/x86/kvm/vmx/vmx.h |  2 ++
 2 files changed, 19 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 41772ecdd368..d00ab9d4c93e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1344,6 +1344,17 @@ 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_cpuid_has(vcpu, X86_FEATURE_FRED)) {
+		/*
+		 * MSR_IA32_FRED_RSP0 is top of task stack, which never changes.
+		 * Thus it should be initialized only once.
+		 */
+		if (unlikely(vmx->msr_host_fred_rsp0 == 0))
+			vmx->msr_host_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
+	}
 #else
 	savesegment(fs, fs_sel);
 	savesegment(gs, gs_sel);
@@ -1388,6 +1399,12 @@ 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_cpuid_has(&vmx->vcpu, X86_FEATURE_FRED)) {
+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_host_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 f8c02bd37069..328a3447f064 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -276,6 +276,8 @@ struct vcpu_vmx {
 #ifdef CONFIG_X86_64
 	u64		      msr_host_kernel_gs_base;
 	u64		      msr_guest_kernel_gs_base;
+	u64		      msr_host_fred_rsp0;
+	u64		      msr_guest_fred_rsp0;
 #endif
 
 	u64		      spec_ctrl;
-- 
2.42.0