Re: [PATCH v5 20/34] x86/fred: add a machine check entry stub for FRED

Peter Zijlstra
2023-03-21T09:02:40Z

On Tue, Mar 21, 2023 at 12:04:47AM +0000, Li, Xin3 wrote:
> > > Unlike IDT, no need to save/restore dr7 in FRED machine check handler.
> >
> > Given how fragile MCE is, the question should be, do we ever want hw
> > breakpoints to happen while it is running?
>
> HW breakpoints still work if they are properly configured.
>
> > If the hw-breakpoint handler trips on the same memory fail that got us into the
> > mce the first time, we're dead.
>
> Right.
>
> Unless the MCIP bit is turned off any subsequent #MC goes to shutdown
> ("machine is screwed").
>
> It's the kernel debugger's responsibility to decide how to proceed in such
> cases. But if the kernel debugger itself is in a screwed memory region, we
> are soooooo dead.

Yeah, so I would much prefer, for robustness sake, to start out with not
allowing #DB in MCE -- much like today.