Re: Finish removing MPX from arch/x86?

From: Dave Hansen
Date: Tue Oct 04 2022 - 17:07:53 EST

On 10/4/22 11:21, Jim Mattson wrote:
> On Tue, Oct 4, 2022 at 10:45 AM Dave Hansen <dave.hansen@xxxxxxxxx> 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.