Re: [PATCH] KVM: VMX: do not disable interception for MSR_IA32_SPEC_CTRL on eIBRS

[Jon Kohler]

> On May 20, 2022, at 4:06 PM, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> 
> On Fri, May 20, 2022, Jon Kohler wrote:
>> 
>>> On May 18, 2022, at 10:23 AM, Jon Kohler <jon@xxxxxxxxxxx> wrote:
>>> 
>>>> On May 17, 2022, at 9:42 PM, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>>>>> +		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) && data == BIT(0)) {
>>>> 
>>>> Use SPEC_CTRL_IBRS instead of open coding "BIT(0)", then a chunk of the comment
>>>> goes away.
>>>> 
>>>>> +			vmx->spec_ctrl = data;
>>>>> +			break;
>>>>> +		}
>>>> 
>>>> There's no need for a separate if statement.  And the boot_cpu_has() check can
>>>> be dropped, kvm_spec_ctrl_test_value() has already verified the bit is writable
>>>> (unless you're worried about bit 0 being used for something else?)
>> 
>> I was (and am) worried about misbehaving guests on pre-eIBRS systems spamming IBRS
>> MSR, which we wouldn’t be able to see today. Intel’s guidance for eIBRS has long been
>> set it once and be done with it, so any eIBRS aware guest should behave nicely with that.
>> That limits the blast radius a bit here.
> 
> Then check the guest capabilities, not the host flag.
> 
> 	if (data == SPEC_CTRL_IBRS &&
> 	    (vcpu->arch.arch_capabilities & ARCH_CAP_IBRS_ALL))

So I originally did that in my first internal patch; however, the code you wrote is
effectively the code I wrote, because cpu_set_bug_bits() already does that exact
same thing when it sets up X86_FEATURE_IBRS_ENHANCED. 

Is the boot cpu check more expensive than checking the vCPU perhaps? Otherwise,
checking X86_FEATURE_IBRS_ENHANCED seemed like it might be easier
understand for future onlookers, as thats what the rest of the kernel keys off of
when checking for eIBRS (e.g. in bugs.c etc). 

>> Sent out the v2 just now with a few minor tweaks, only notable one was keeping
>> the boot cpu check and small tweaks to comments here and there to suit.
> 
> In the future, give reviewers a bit of time to respond to a contented point before
> sending out the next revision, e.g. you could have avoided v3 :-)

Thanks for the feedback/coaching. Still getting my legs under me for LKML, I
appreciate the kindness, thank you!