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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Nov 2019 11:54:15 -0800
Message-ID: <CAHk-=wiGO=+mmEb-sfCsD5mxmL5++gdwkFj_aXcfz1R41MJnEg@mail.gmail.com>
Subject: Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further
To:     Brian Gerst <brgerst@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Willy Tarreau <w@1wt.eu>, Juergen Gross <jgross@suse.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Thu, Nov 7, 2019 at 11:24 AM Brian Gerst <brgerst@gmail.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).

               Linus