Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover from machine checks

Andy Lutomirski
2015-12-31T21:22:42Z

On Jan 1, 2016 4:30 AM, "Tony Luck" <tony.luck@gmail.com> wrote:
>
> On Wed, Dec 30, 2015 at 3:32 PM, Tony Luck <tony.luck@gmail.com> wrote:
> > Fifth is just a hack because I clearly didn't understand what I was
> > doing in parts 2&3 because my new class shows up as '3' not '1'!
> >
> > Andy: Can you explain the assembler/linker arithmetic for the class?
>
> Never mind ... figured it out.
>
> The fixup entry in the extable is:
>
> label - . + 0x2000000 - BIAS
>
> The "label - ." part evaluates to a smallish negative value (because
> the .fixup section is bundled in towards the end of .text, and the
> ex_table section comes right after.
>
> Then you add 0x20000000 to get a positive number, then *subtract*
> the BIAS. I'd picked BIAS = 0x40000000 thinking that would show
> up directly in class bits. But 0x1ffff000 - 0x40000000 is 0xdffff000
> so bits 31 & 31 are both set, and this is class3
>
> I switched to BIAS 0xC0000000 ... and now I get class 1 entries
> (bit31=0, bit30=1).
>
> New patch series coming soon.

That all sounds correct.

You could also just to s/UACCESS/INDIRECT/ or whatever and leave using
the next bit for whoever does the uaccess part, too. After all,
introducing the "uaccess" class without actually implementing it isn't
very useful.

>
> -Tony