Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

[Brian Gerst]

On Thu, Nov 7, 2019 at 2:54 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 7, 2019 at 11:24 AM Brian Gerst <brgerst@xxxxxxxxx> wrote:
> >
> > Here is a different idea:  We already map the TSS virtually in
> > cpu_entry_area.  Why not page-align the IO bitmap and remap it to the
> > task's bitmap on task switch?  That would avoid all copying on task
> > switch.
>
> We map the tss _once_, statically, percpu, without ever changing it,
> and then we just (potentially) change a couple of fields in it on
> process switch.
>
> Your idea isn't horrible, but it would involve a TLB flush for the
> page when the io bitmap changes. Which is almost certainly more
> expensive than just copying the bitmap intelligently.
>
> Particularly since I do think that the copy can basically be done
> effectively never, assuming there really aren't multiple concurrent
> users of ioperm() (and iopl).

There wouldn't have to be a flush on every task switch.  If we make it
so that tasks that don't use a bitmap just unmap the pages in the
cpu_entry_area and set tss.io_bitmap_base to outside the segment
limit, we would only have to flush when switching from a task using
the bitmap (because the next task uses a different bitmap or we are
unmapping it).  If the previous task doesn't have a bitmap the pages
in cpu_entry_area were unmapped and can't be in the TLB, so no flush
is needed.

Going a step further, we could track which task is mapped to the
current cpu like proposed above, and only flush when a different task
needs the IO bitmap, or when the bitmap is being freed on task exit.

--
Brian Gerst