Hardware does not save/restore FRED_RSP0 for Non-SEV-ES guests. Save it
in svm_prepare_host_switch() and restore it in svm_prepare_switch_to_guest()
so that the correct physical CPU state is updated.

Also, synchronize the current value of MSR_IA32_FRED_RSP0 in hardware to the
kernel's local cache. Note that the desired host's RSP0 will be set when the
CPU exits to userspace for servicing vCPU tasks.

Co-developed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
---

Changes in v3:
* Keep the guest's FRED_RSP0 in a new vcpu_svm field, msr_guest_fred_rsp0.
  The VMCB has no FRED_RSP0 field for non-SEV-ES guests.
* Zero msr_guest_fred_rsp0 on RESET.
* Added the svm_manages_fred_rsp0() helper.

Changes in v2:
* Moved FRED MSRs save/restore logic from svm_vcpu_enter_exit() to
  svm_prepare_[host_switch/switch_to_guest]() to reduce some MSR accesses.
* While switching to host, added a safety check on guest_state_loaded.

---
 arch/x86/kvm/svm/svm.c | 28 +++++++++++++++++++++++++++-
 arch/x86/kvm/svm/svm.h |  2 ++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e212f53d262a..c1263a3b3e73 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1234,6 +1234,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu, bool init_event)
 	save->idtr.limit = 0xffff;
 
 	if (!init_event) {
+		svm->msr_guest_fred_rsp0 = 0;
 		save->fred_rsp1 = 0;
 		save->fred_rsp2 = 0;
 		save->fred_rsp3 = 0;
@@ -1458,6 +1459,11 @@ static void svm_srso_vm_init(void) { }
 static void svm_srso_vm_destroy(void) { }
 #endif
 
+static bool svm_manages_fred_rsp0(struct kvm_vcpu *vcpu)
+{
+	return !is_sev_es_guest(vcpu) && guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);
+}
+
 static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -1495,12 +1501,32 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 		sd->bp_spec_reduce_set = true;
 		msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
 	}
+
+	/* Hardware does not save/restore FRED_RSP0 for Non-SEV-ES guests. */
+	if (svm_manages_fred_rsp0(vcpu))
+		wrmsrq(MSR_IA32_FRED_RSP0, svm->msr_guest_fred_rsp0);
+
 	svm->guest_state_loaded = true;
 }
 
 static void svm_prepare_host_switch(struct kvm_vcpu *vcpu)
 {
-	to_svm(vcpu)->guest_state_loaded = false;
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	if (!svm->guest_state_loaded)
+		return;
+
+	/*
+	 * Hardware does not save/restore FRED_RSP0 for Non-SEV-ES guests.
+	 * Also, sync hardware MSR value to per-CPU cache. This helps in
+	 * restoring Host RSP0 when exiting to userspace in fred_update_rsp0().
+	 */
+	if (svm_manages_fred_rsp0(vcpu)) {
+		rdmsrq(MSR_IA32_FRED_RSP0, svm->msr_guest_fred_rsp0);
+		fred_sync_rsp0(svm->msr_guest_fred_rsp0);
+	}
+
+	svm->guest_state_loaded = false;
 }
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index e958943b8162..581289cc6e0b 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -362,6 +362,8 @@ struct vcpu_svm {
 
 	bool guest_state_loaded;
 
+	u64 msr_guest_fred_rsp0;
+
 	bool avic_irq_window;
 	bool x2avic_msrs_intercepted;
 	bool lbr_msrs_intercepted;
-- 
2.43.0