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

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Date:   Thu, 07 Nov 2019 15:20:15 -0800
From:   hpa@...or.com
To:     Thomas Gleixner <tglx@...utronix.de>,
        Brian Gerst <brgerst@...il.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

On November 7, 2019 1:32:15 PM PST, Thomas Gleixner <tglx@...utronix.de> wrote:
>On Thu, 7 Nov 2019, Brian Gerst wrote:
>> On Thu, Nov 7, 2019 at 2:54 PM Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>> >
>> > On Thu, Nov 7, 2019 at 11:24 AM Brian Gerst <brgerst@...il.com>
>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.
>
>Funny. I was just debating exactly this with Peter Ziljstra over IRC :)
> 
>> 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.
>
>Yes.
>
>But, we really should check what aside of DoSemu is using this still.
>None
>of my machines I checked have a single instance of ioperm()/iopl()
>usage.
>
>So the real question is whether it's worth the trouble or if we are
>just
>better off to copy if there is an actual user and the sequence count of
>the
>bitmap is different than the one which was active last time.
>
>Thanks,
>
>	tglx

I have written suffer using this, because of far better real time performance. I just want to punch a hole (just like mmapping an MMIO device.)

I do agree that let's not optimize for the rare case.
-- 
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