linux-kernel - Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Tue, 9 Jan 2018 11:11:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, aliguori@...zon.com,
        Tom Lendacky <thomas.lendacky@....com>, dwmw@...zon.co.uk,
        bp@...en8.de
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On 09/01/2018 00:19, Jim Mattson wrote:
>>>> +       vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
>>>> +       vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
>>> I have a lot of changes to MSR permission bitmap handling, but these
>>> intercepts should only be disabled when guest_cpuid_has(vcpu,
>>> X86_FEATURE_SPEC_CTRL).
>> That's harder to backport and not strictly necessary (just like
>> e.g. we don't check guest CPUID bits before emulation).  I agree that
>> your version is better, but I think the above is fine as a minimal
>> fix.
> 
> Due to the impacts that spec_ctrl has on the neighboring hyperthread,
> one may want to disable MSRs 0x48 and 0x49 for a particular VM. We do
> this by masking off CPUID.(EAX=7, ECX=0).EDX[26] and CPUID.(EAX=7,
> ECX=0).EDX[27] from the userspace agent. However, with your patch,
> *any* VCPU gets unrestricted access to these MSRs, without any
> mechanism for disabling them.

Yes, I agree that having the check is superior.  However, I also want to
get there step by step.

>>>> +       if (have_spec_ctrl) {
>>>> +               rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>>>> +               if (vmx->spec_ctrl)
>>>> +                       wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>>>> +       }
>>>> +
>>>
>>> I know the VM-exit MSR load and store lists are probably slower, but
>>> I'm a little uncomfortable restoring the host's IA32_SPEC_CTRL MSR
>>> late if the guest has it clear and the host has it set.
>>
>> There is no indirect branch before though, isn't it?
>
> I guess that depends on how you define "before."

--verbose? :-/

Paolo