linux-kernel - Re: [PATCH 2/2] KVM: x86: always expose VIRT

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Message-ID: <dadab484-68f8-1588-534b-c2a90c3b7fe4@redhat.com>
Date:   Mon, 19 Aug 2019 17:41:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 2/2] KVM: x86: always expose VIRT_SSBD to guests

On 15/08/19 19:14, Eduardo Habkost wrote:
> On Thu, Aug 15, 2019 at 09:41:23AM +0200, Paolo Bonzini wrote:
>> Even though it is preferrable to use SPEC_CTRL (represented by
>> X86_FEATURE_AMD_SSBD) instead of VIRT_SPEC, VIRT_SPEC is always
>> supported anyway because otherwise it would be impossible to
>> migrate from old to new CPUs.  Make this apparent in the
>> result of KVM_GET_SUPPORTED_CPUID as well.
>>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>> Reported-by: Eduardo Habkost <ehabkost@...hat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/kvm/cpuid.c | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 145ec050d45d..5865bc73bbb5 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -747,11 +747,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>>  		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
>>  		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
>>  		/*
>> -		 * The preference is to use SPEC CTRL MSR instead of the
>> -		 * VIRT_SPEC MSR.
>> +		 * VIRT_SPEC is only implemented for AMD processors,
>> +		 * but the host could set AMD_SSBD if it wanted even
>> +		 * for Intel processors.
>>  		 */
>> -		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
>> -		    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
>> +		if ((boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
>> +		     boot_cpu_has(X86_FEATURE_AMD_SSBD)) &&
>> +		    boot_cpu_has(X86_FEATURE_SVM))
> 
> Would it be desirable to move this code to
> svm_set_supported_cpuid(), or is there a reason for keeping this
> in cpuid.c?

Yes, of course.  Forgot about it.

Paolo