linux-kernel - Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection

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Date:   Wed, 29 Jul 2020 23:09:35 +0200
From:   peterz@...radead.org
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>, H Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection

On Wed, Jul 29, 2020 at 08:40:57PM +0000, Fenghua Yu wrote:
> On Wed, Jul 29, 2020 at 10:49:47AM +0200, peterz@...radead.org wrote:
> > On Fri, Jul 17, 2020 at 02:35:00PM -0700, Fenghua Yu wrote:
> > 
> > > #DB for bus lock detect fixes all issues in #AC for split lock detect:
> > > 1) It's architectural ... just need to look at one CPUID bit to know it
> > >    exists
> > > 2) The IA32_DEBUGCTL MSR, which reports bus lock in #DB, is per-thread.
> > >    So each process or guest can have different behavior.
> > 
> > And it generates a whole new problem due to #DB being an IST, and

> > we very much rely on #DB never recursing, which we carefully crafted by
> > disallowing hardare breakpoints on noinstr code and clearing DR7 early.
> > 
> > But now it can... please keep the pieces.
> 
> Can we disable Bus Lock Detection before handle it and re-enable it
> after handle it? Will that resolve the recursion issue?

Because WRMSR is cheap, right?

You have to unconditionally {dis,en}able it on #DB entry/exit. Not only
when it's a DR_BUS_LOCK, _always_. Then maybe. I'm too tired to think
through the IST mess.

IST's suck, they're horrible crap.

Suppose we get a #DB, then we get an NMI right before it does WRMSR, so
BUS_LOCK is still on, then the NMI does a dodgy LOCK op, we die.

So that means, you get to disable it on every NMI-like exception too,
but we happen to care about performance for those, you loose.


Also, what happens if you have a hardware watchpoint on the instruction
that causes DR_BUS_LOCK? Does that work as expected?