Hi, Peter,
On Wed, Jul 29, 2020 at 10:49:47AM +0200, pet...@infradead.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
>
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index b038695f36c5..58725567da39 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -812,6 +812,16 @@ static void handle_debug(struct pt_regs *regs,
> > unsigned long dr6, bool user)
> > if (!user && !dr6)
> > return;
> >
> > + /* Handle bus lock. */
> > + if (!(dr6 & DR_BUS_LOCK)) {
> > + cond_local_irq_enable(regs);
> > + if (user)
> > + handle_user_bus_lock(regs);
> > + else
> > + handle_kernel_bus_lock(regs);
> > + goto out;
> > + }
> > +
> > /*
> > * If dr6 has no reason to give us about the origin of this trap,
> > * then it's very likely the result of an icebp/int01 trap.
>
> 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?
Thanks.
-Fenghua
