linux-kernel - Re: [patch V3 12/20] x86/ioperm: Move TSS bitmap update to exit to user work

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Date:   Wed, 13 Nov 2019 13:19:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V3 12/20] x86/ioperm: Move TSS bitmap update to exit to
 user work

On Wed, Nov 13, 2019 at 1:02 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> From: Thomas Gleixner <tglx@...utronix.de>
>
> There is no point to update the TSS bitmap for tasks which use I/O bitmaps
> on every context switch. It's enough to update it right before exiting to
> user space.

Hmm.

I wonder if it might make sense to delay it even more: just always
invalidate the bitmap on task switch, and leave it at that.

And then on GP fault, just add trivial logic like

     if (I_have_an_io_bitmap && it_isnt_installed) {
          install_io_bitmap();
          return;
     }

and now you do get that extra GP fault if you actually use IO
accesses, but the normal case has zero overhead.

Even processes that do ioperm may not be *using* it all the time.

               Linus