linux-kernel - Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid

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Date:   Tue, 7 Jul 2020 19:26:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid_bits

On 07/07/20 13:30, Maxim Levitsky wrote:
>> Somehwat crazy idea inbound... rather than calculating the valid bits in
>> software, what if we throw the value at the CPU and see if it fails?  At
>> least that way the host and guest are subject to the same rules.  E.g.
>>
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2062,11 +2062,19 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>                     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
>>                         return 1;
>>
>> -               if (data & ~kvm_spec_ctrl_valid_bits(vcpu))
>> -                       return 1;
>> -
>> +               ret = 0;
>>                 vmx->spec_ctrl = data;
>> -               if (!data)
>> +
>> +               local_irq_disable();
>> +               if (rdmsrl_safe(MSR_IA32_SPEC_CTRL, &data))
>> +                       ret = 1;
>> +               else if (wrmsrl_safe(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl))
>> +                       ret = 1;
>> +               else
>> +                       wrmsrl(MSR_IA32_SPEC_CTRL, data))
>> +               local_irq_enable();
>> +
>> +               if (ret || !vmx->spec_ctrl)
>>                         break;
>>
>>                 /*
>>
> I don't mind this as well, knowing that this is done only one per VM run anyway.

Maxim, this is okay as well; can you send a patch for it?

Paolo