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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Nov 6, 2019 at 12:57 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Calculate both the position of the first zero bit and the last zero bit to
> limit the range which needs to be copied. This does not solve the problem
> when the previous tasked had only byte 0 cleared and the next one has only
> byte 65535 cleared, but trying to solve that would be too complex and
> heavyweight for the context switch path. As the ioperm() usage is very rare
> the case which is optimized is the single task/process which uses ioperm().

Hmm.

I may read this patch wrong, but from what I can tell, if we really
have just one process with an io bitmap, we're doing unnecessary
copies.

If we really have just one process that has an iobitmap, I think we
could just keep the bitmap of that process entirely unchanged. Then,
when we switch away from it, we set the io_bitmap_base to an invalid
base outside the TSS segment, and when we switch back, we set it back
to the valid one. No actual bitmap copies at all.

So I think that rather than the "begin/end offset" games, we should
perhaps have a "what was the last process that used the IO bitmap for
this TSS" pointer (and, I think, some sequence counter, so that when
the process updates its bitmap, it invalidates that case)?

 Of course, you can do *nboth*, but if we really think that the common
case is "one special process", then I think the begin/end offset is
useless, but a "last bitmap process" would be very useful.

Am I missing something?

               Linus