Re: [PATCH 7/7] KVM: SVM: Enable save/restore of FRED MSRs

[Shivansh Dhiman]

On 07-03-2026 07:44, Sean Christopherson wrote:
> On Thu, Jan 29, 2026, Shivansh Dhiman wrote:
>> Set the FRED_VIRT_ENABLE bit (bit 4) in the VIRT_EXT field of VMCB to enable
>> FRED Virtualization for the guest. This enables automatic save/restore of
>> FRED MSRs. Also toggle this bit when setting CPUIDs, to support booting of
>> secure guests.
>>
>> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@xxxxxxx>
>> ---
>>  arch/x86/kvm/svm/svm.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 954df4eae90e..24579c149937 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -1144,6 +1144,9 @@ static void init_vmcb(struct kvm_vcpu *vcpu, bool init_event)
>>  	save->fred_ssp3 = 0;
>>  	save->fred_config = 0;
>>  
>> +	if (guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
>> +		svm->vmcb->control.virt_ext |= FRED_VIRT_ENABLE_MASK;
> 
> This is completely unnecessary, no?  CPUID is empty at vCPU creation and so FRED
> _can't_ be enabled before going through svm_vcpu_after_set_cpuid().

On a second thought, this is actually redundant. I'll drop this from init_vmcb().

> 
>>  	init_sys_seg(&save->ldtr, SEG_TYPE_LDT);
>>  	init_sys_seg(&save->tr, SEG_TYPE_BUSY_TSS16);
>>  
>> @@ -4529,6 +4532,9 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>>  	if (guest_cpuid_is_intel_compatible(vcpu))
>>  		guest_cpu_cap_clear(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
>>  
>> +	if (guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
>> +		svm->vmcb->control.virt_ext |= FRED_VIRT_ENABLE_MASK;
> 
> The flag needs to be cleared if FRED isn't supported, because KVM's wonderful
> ABI allows userspace to modify CPUID however many times it wants before running
> the vCPU.

Agreed. I'll add:

if (guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
	svm->vmcb->control.virt_ext |= FRED_VIRT_ENABLE_MASK;
else
	svm->vmcb->control.virt_ext &= ~FRED_VIRT_ENABLE_MASK;

- Shivansh

> 
>> +
>>  	if (sev_guest(vcpu->kvm))
>>  		sev_vcpu_after_set_cpuid(svm);
>>  }
>> -- 
>> 2.43.0
>>