Re: [Xen-devel] [PATCH 1/2] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> This demotes an OOPS and likely panic due to a failed non-"safe" MSR
> access to a WARN_ON_ONCE and a return of poisoned values (in the
> RDMSR case).  We still write a pr_info entry unconditionally for
> debugging.
> 
> To be clear, this type of failure should *not* happen.  This patch
> exists to minimize the chance of nasty undebuggable failures due on
> systems that used to work due to a now-fixed CONFIG_PARAVIRT=y bug.

> +     if (opcode == 0x320f) {
> +             /* RDMSR */
> +             pr_info("bad kernel RDMSR from non-existent MSR 0x%x",
> +                     (unsigned int)regs->cx);
> +             if (!panic_on_oops) {
> +                     WARN_ON_ONCE(true);
> +
> +                     /* Patch it up with deterministic poison. */
> +                     regs->ax = 0x5aadc0de;
> +                     regs->dx = 0x8badf00d;
> +                     regs->ip += 2;
> +                     return true;

IMHO this should really not poison the result, but use zero as the result.

The poison might randomly indicate 'present' feature in various registers that 
might be accessed in a buggy way. Don't send the code further down into 
la-la-land 
by giving it a 'success'.

And yes, zero can mean success too, but we have to pick a side here ...

The warning will be enough to fix these ups, people (and in particular distro 
testing people) will be watching out for them.

Thanks,

        Ingo

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