linux-kernel - Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more

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Message-ID: <20200525180834.GF317569@hirez.programming.kicks-ass.net>
Date:   Mon, 25 May 2020 20:08:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more

On Mon, May 25, 2020 at 10:19:08AM -0700, Andy Lutomirski wrote:

> How about adding it to cpu_tlbstate?  A lot of NMIs are going to read
> that anyway to check CR3.

That might work I suppose; we're really pushing the name of it though.
Also, that's PTI specific IIRC, and we're getting to the point where a
significant number of CPUs no longer need that, right?

> And blaming KVM is a bit misplaced. This isn’t KVM’s fault — it’s
> Intel’s. VT-x has two modes: DR access exits and DR access doesn’t
> exit. There’s no shadow mode.

It's virt, I can't be arsed to care, whoever misdesigned it.
We already have debugreg pvops, they can do shadow there.