Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 6 Nov 2019 17:11:15 -0800 | Subject | Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further |
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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
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