linux-kernel - Re: Finish removing MPX from arch/x86?

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Date:   Tue, 4 Oct 2022 14:07:45 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: Finish removing MPX from arch/x86?

On 10/4/22 11:21, Jim Mattson wrote:
> On Tue, Oct 4, 2022 at 10:45 AM Dave Hansen <dave.hansen@...el.com> wrote:
>>
>> We zapped the userspace MPX ABIs and most of its supporting code in here:
>>
>>         45fc24e89b7c ("x86/mpx: remove MPX from arch/x86")
>>
>> But, the XSAVE enabling and KVM code were left in place.  This let folks
>> at least keep running guests with MPX.
>>
>> It's been a couple of years and I don't think I've had a single person
>> opine about the loss of MPX.  Intel also followed through and there's no
>> MPX to be found on newer CPUs like my "Tiger Lake" 11th Gen Intel(R)
>> Core(TM) i7-1165G7.
>>
>> Is it time to zap MPX from arch/x86/kvm/?
> 
> Until Google Cloud retires all of our MPX-capable hardware, we will
> require MPX support in KVM.
> 
> Removing that support would leave VMs with MPX enabled in XCR0 with
> nowhere to go.

Is this because you migrate guest VMs between hosts?  A _potential_ VM
migration target host is ineligible if it has a subset of the features
of the source host?

Would it be _possible_ to leave existing VMs alone, but to stop
enumerating MPX to newly-created VMs?  I don't know how long-lived your
VMs are, but I'm hoping that the ones that know about MPX will all die
naturally of old age at some point.