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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 24 Oct 2019 21:45:56 -0700
Message-ID: <CALCETrVepdYd4uN8jrG8i6iaixWp+N3MdGv5WhjOdCr9sLRK1w@mail.gmail.com>
Subject: Please stop using iopl() in DPDK
To:     dev@dpdk.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi all-

Supporting iopl() in the Linux kernel is becoming a maintainability
problem.  As far as I know, DPDK is the only major modern user of
iopl().

After doing some research, DPDK uses direct io port access for only a
single purpose: accessing legacy virtio configuration structures.
These structures are mapped in IO space in BAR 0 on legacy virtio
devices.

There are at least three ways you could avoid using iopl().  Here they
are in rough order of quality in my opinion:

1. Change pci_uio_ioport_read() and pci_uio_ioport_write() to use
read() and write() on resource0 in sysfs.

2. Use the alternative access mechanism in the virtio legacy spec:
there is a way to access all of these structures via configuration
space.

3. Use ioperm() instead of iopl().


We are considering changes to the kernel that will potentially harm
the performance of any program that uses iopl(3) -- in particular,
context switches will become more expensive, and the scheduler might
need to explicitly penalize such programs to ensure fairness.  Using
ioperm() already hurts performance, and the proposed changes to iopl()
will make it even worse.  Alternatively, the kernel could drop iopl()
support entirely.  I will certainly make a change to allow
distributions to remove iopl() support entirely from their kernels,
and I expect that distributions will do this.

Please fix DPDK.

Thanks,
Andy