linux-kernel - Re: [PATCH v2 1/7] KVM: X86: Reset vcpu->arch.cpuid_nent to 0 if SET

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Message-ID: <59535ddb-373a-952f-f049-91d8f142c6a9@intel.com>
Date:   Wed, 24 Jun 2020 08:40:11 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/7] KVM: X86: Reset vcpu->arch.cpuid_nent to 0 if
 SET_CPUID fails

On 6/24/2020 2:20 AM, Jim Mattson wrote:
> On Tue, Jun 23, 2020 at 4:58 AM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
>>
>> It needs to invalidate CPUID configruations if usersapce provides
> 
> Nits: configurations, userspace

oh, I'll fix it.

>> illegal input.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 8a294f9747aa..1d13bad42bf9 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -207,6 +207,8 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
>>          kvm_apic_set_version(vcpu);
>>          kvm_x86_ops.cpuid_update(vcpu);
>>          r = kvm_update_cpuid(vcpu);
>> +       if (r)
>> +               vcpu->arch.cpuid_nent = 0;
>>
>>          kvfree(cpuid_entries);
>>   out:
>> @@ -230,6 +232,8 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
>>          kvm_apic_set_version(vcpu);
>>          kvm_x86_ops.cpuid_update(vcpu);
>>          r = kvm_update_cpuid(vcpu);
>> +       if (r)
>> +               vcpu->arch.cpuid_nent = 0;
>>   out:
>>          return r;
>>   }
>> --
>> 2.18.2
> 
> What if vcpu->arch.cpuid_nent was greater than 0 before the ioctl in question?
>

Nice catch!

If considering it, then we have to restore the old CPUID configuration. 
So how about making it simpler to just add one line of comment in API doc:
If KVM_SET_CPUID{2} fails, the old valid configuration is cleared as a 
side effect.