Re: [PATCH 3/7] KVM: SVM: Save restore FRED_RSP0 for FRED supported guests

[Shivansh Dhiman]

Hi,

On 29-01-2026 12:06, Shivansh Dhiman wrote:
> From: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> 
> 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.


I'm planning to improve this path in v2 of this series, by moving the
restoring/saving of FRED RSP0 to svm_prepare_[switch_to_guest/host_switch]
respectively, thus saving some MSR accesses. Any comments are welcome.

> 
> 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@xxxxxxx>
> Co-developed-by: Shivansh Dhiman <shivansh.dhiman@xxxxxxx>
> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@xxxxxxx>
> ---
>  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();
>  

@@ -1391,12 +1433,34 @@ 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 (!sev_es_guest(vcpu->kvm) && guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+               wrmsrq(MSR_IA32_FRED_RSP0, svm->vmcb->save.fred_rsp0);
+
        svm->guest_state_loaded = true;
 }

> @@ -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)

 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 (!sev_es_guest(vcpu->kvm) && guest_cpu_cap_has(vcpu, X86_FEATURE_FRED)) {
+               rdmsrq(MSR_IA32_FRED_RSP0, svm->vmcb->save.fred_rsp0);
+               fred_sync_rsp0(svm->vmcb->save.fred_rsp0);
+       }
+
+       svm->guest_state_loaded = false;
 }

Thanks,
Shivansh