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

From: Willy Tarreau <w@1wt.eu>
To: hpa@zytor.com
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Juergen Gross <jgross@suse.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further
Date: Thu, 7 Nov 2019 11:27:56 +0100	[thread overview]
Message-ID: <20191107102756.GD15536@1wt.eu> (raw)
In-Reply-To: <71DE81AC-3AD4-47B3-9CBA-A2C7841A3370@zytor.com>

On Thu, Nov 07, 2019 at 02:19:19AM -0800, hpa@zytor.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