Re: [PATCH v3 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL

[Babu Moger]

On 1/19/21 12:31 PM, Sean Christopherson wrote:
> On Fri, Jan 15, 2021, Babu Moger wrote:
>> ---
>>  arch/x86/include/asm/svm.h |    4 +++-
>>  arch/x86/kvm/svm/sev.c     |    4 ++++
>>  arch/x86/kvm/svm/svm.c     |   19 +++++++++++++++----
>>  3 files changed, 22 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
>> index 1c561945b426..772e60efe243 100644
>> --- a/arch/x86/include/asm/svm.h
>> +++ b/arch/x86/include/asm/svm.h
>> @@ -269,7 +269,9 @@ struct vmcb_save_area {
>>  	 * SEV-ES guests when referenced through the GHCB or for
>>  	 * saving to the host save area.
>>  	 */
>> -	u8 reserved_7[80];
>> +	u8 reserved_7[72];
>> +	u32 spec_ctrl;		/* Guest version of SPEC_CTRL at 0x2E0 */
>> +	u8 reserved_7b[4];
> 
> Don't nested_prepare_vmcb_save() and nested_vmcb_checks() need to be updated to
> handle the new field, too?

Ok. Sure. I will check and test few combinations to make sure of these
changes.

> 
>>  	u32 pkru;
>>  	u8 reserved_7a[20];
>>  	u64 reserved_8;		/* rax already available at 0x01f8 */
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index c8ffdbc81709..959d6e47bd84 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -546,6 +546,10 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>>  	save->pkru = svm->vcpu.arch.pkru;
>>  	save->xss  = svm->vcpu.arch.ia32_xss;
>>  
>> +	/* Update the guest SPEC_CTRL value in the save area */
>> +	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>> +		save->spec_ctrl = svm->spec_ctrl;
> 
> I think this can be dropped if svm->spec_ctrl is unused when V_SPEC_CTRL is
> supported (see below).  IIUC, the memcpy() that's just out of sight would do
> the propgation to the VMSA.

Yes, That is right. I will remove this.

> 
>> +
>>  	/*
>>  	 * SEV-ES will use a VMSA that is pointed to by the VMCB, not
>>  	 * the traditional VMSA that is part of the VMCB. Copy the
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 7ef171790d02..a0cb01a5c8c5 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -1244,6 +1244,9 @@ static void init_vmcb(struct vcpu_svm *svm)
>>  
>>  	svm_check_invpcid(svm);
>>  
>> +	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>> +		save->spec_ctrl = svm->spec_ctrl;
>> +
>>  	if (kvm_vcpu_apicv_active(&svm->vcpu))
>>  		avic_init_vmcb(svm);
>>  
>> @@ -3789,7 +3792,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>>  	 * is no need to worry about the conditional branch over the wrmsr
>>  	 * being speculatively taken.
>>  	 */
>> -	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
>> +	if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>> +		svm->vmcb->save.spec_ctrl = svm->spec_ctrl;
>> +	else
>> +		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
> 
> Can't we avoid functional code in svm_vcpu_run() entirely when V_SPEC_CTRL is
> supported?  Make this code a nop, disable interception from time zero, and

Sean, I thought you mentioned earlier about not changing the interception
mechanism. Do you think we should disable the interception right away if
V_SPEC_CTRL is supported?

> read/write the VMBC field in svm_{get,set}_msr().  I.e. don't touch
> svm->spec_ctrl if V_SPEC_CTRL is supported.  
> 
> 	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> 		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
> 
> 	svm_vcpu_enter_exit(vcpu, svm);
> 
> 	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL) &&
> 	    unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
> 		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);

Ok. It appears the above code might work fine with changes in
svm_{get,set}_msr() to update save spec_ctlr. I will retest few
combinations to make sure it works.
Thanks
Babu

> 
>>  	svm_vcpu_enter_exit(vcpu, svm);
>>  
>> @@ -3808,13 +3814,18 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>>  	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
>>  	 * save it.
>>  	 */
>> -	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
>> -		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
>> +	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))) {
>> +		if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>> +			svm->spec_ctrl = svm->vmcb->save.spec_ctrl;
>> +		else
>> +			svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
>> +	}
>>  
>>  	if (!sev_es_guest(svm->vcpu.kvm))
>>  		reload_tss(vcpu);
>>  
>> -	x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
>> +	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>> +		x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
>>  
>>  	if (!sev_es_guest(svm->vcpu.kvm)) {
>>  		vcpu->arch.cr2 = svm->vmcb->save.cr2;
>>