Re: [PATCH] x86/mce: Fix set_mce_nospec() to avoid #GP fault

[Tony Luck]

On Thu, Aug 30, 2018 at 6:30 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 30, 2018 at 2:45 PM Tony Luck <tony.luck@xxxxxxxxx> wrote:
> >
> > Fix is to move one step at a time. First mark the page not present
> > (using the decoy address). Then it is safe to use the actual address
> > of the 1:1 mapping to mark it "uc", and finally as present.
>
> Can't we do it in one step, but make sure that he clflush gets the real address?
>

I'd like to, but I'd need to have a way to mark the address as needing fixing as
it gets passed from set_memory_uc() to _set_memory_uc() to
change_page_attr_set()
to change_page_attr_set_clear()

Just checking "do we have a non-canonical address" at the bottom of that
call stack and flipping bit 63 back on again seems like a bad idea. But adding
extra flag arguments is majorly ugly too.

-Tony