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

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Date:   Thu, 7 Nov 2019 14:24:12 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

On Wed, Nov 6, 2019 at 4:01 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Sane ioperm() users only set the few bits in the I/O space which they need to
> access. But the update logic of the TSS I/O bitmap copies always from byte
> 0 to the last byte in the tasks bitmap which contains a zero permission bit.
>
> That means that for access only to port 65335 the full 8K bitmap has to be
> copied even if all the bytes in the TSS I/O bitmap are already filled with
> 0xff.
>
> 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().

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.

--
Brian Gerst