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

[peterz]

On Wed, Jul 29, 2020 at 08:40:57PM +0000, Fenghua Yu wrote:
> On Wed, Jul 29, 2020 at 10:49:47AM +0200, peterz@xxxxxxxxxxxxx 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?