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

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Date:   Wed, 5 Oct 2022 14:04:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.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 19:34, Dave Hansen 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/?

I agree that the likelihood of anybody complaining about MPX is low but 
Jim is right that the timeline for removing it is unfortunately quite long.

Removing MPX from XFEATURE_MASK_USER_SUPPORTED is possible, though we 
need to add a new XFEATURE_MASK_GUEST_SUPPORTED that includes MPX.  I'll 
take a look.

Also, it's worth noting that MPX lives in the sigcontext ABI because it 
uses the non-compacted format.  Because of that it is not possible to 
remove the structs from include/asm/fpu/types.h, for example.

Paolo