linux-kernel - [PATCH 3/7] KVM: SVM: Save restore FRED

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Message-ID: <20260129063653.3553076-4-shivansh.dhiman@amd.com>
Date: Thu, 29 Jan 2026 06:36:49 +0000
From: Shivansh Dhiman <shivansh.dhiman@....com>
To: <seanjc@...gle.com>, <pbonzini@...hat.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC: <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>,
	<xin@...or.com>, <nikunj.dadhania@....com>, <santosh.shukla@....com>
Subject: [PATCH 3/7] KVM: SVM: Save restore FRED_RSP0 for FRED supported guests

From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>

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

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.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Co-developed-by: Shivansh Dhiman <shivansh.dhiman@....com>
Signed-off-by: Shivansh Dhiman <shivansh.dhiman@....com>
---
 arch/x86/kvm/svm/svm.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 05e44e804aba..ddd8941af6f0 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4192,6 +4192,15 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in
 {
 	struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, vcpu->cpu);
 	struct vcpu_svm *svm = to_svm(vcpu);
+	bool update_fred_rsp0;
+
+	/*
+	 * Hardware does not save/restore FRED_RSP0 for Non-SEV-ES guests.
+	 */
+	update_fred_rsp0 = !sev_es_guest(vcpu->kvm) && guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);
+
+	if (update_fred_rsp0)
+		wrmsrq(MSR_IA32_FRED_RSP0, svm->vmcb->save.fred_rsp0);
 
 	guest_state_enter_irqoff();
 
@@ -4218,6 +4227,15 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in
 	raw_local_irq_disable();
 
 	guest_state_exit_irqoff();
+
+	if (update_fred_rsp0) {
+		rdmsrq(MSR_IA32_FRED_RSP0, svm->vmcb->save.fred_rsp0);
+		/*
+		 * Sync hardware MSR value to per-CPU cache. This helps in restoring
+		 * Host RSP0 when exiting to userspace in fred_update_rsp0().
+		 */
+		fred_sync_rsp0(svm->vmcb->save.fred_rsp0);
+	}
 }
 
 static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
-- 
2.43.0