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

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Message-ID: <20191107102756.GD15536@1wt.eu>
Date:   Thu, 7 Nov 2019 11:27:56 +0100
From:   Willy Tarreau <w@....eu>
To:     hpa@...or.com
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        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 Thu, Nov 07, 2019 at 02:19:19AM -0800, hpa@...or.com wrote:
> >Changing ioperm(single port, port range) to be ioperm(all) is going to
> >break a bunch of test cases which actually check whether the permission
> >is restricted to a single I/O port or the requested port range.
> >
> >Thanks,
> >
> >	tglx
> 
> This seems very undesirable... as much as we might wish otherwise, the port
> bitmap is the equivalent to the MMU, and there are definitely users doing
> direct device I/O out there.

Doing these, sure, but doing these while ranges are really checked ?
I mean, the MMU grants you access to the pages you were assigned. Here
with the I/O bitmap you just have to ask for access to port X and you
get it. I could understand the benefit if we had EBUSY in return but
that's not the case, you can actually request access to a port range
another device driver or process is currently using, and mess up with
what it does even by accident. I remember streaming 1-bit music in
userland from the LED of my floppy drive in the late-90s, it used to
cause some trouble to the floppy driver when using mtools in parallel :-)

Willy