linux-kernel - Re: Please stop using iopl() in DPDK

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Message-ID: <20191025064225.GA22917@1wt.eu>
Date:   Fri, 25 Oct 2019 08:42:25 +0200
From:   Willy Tarreau <w@....eu>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     dev@...k.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Please stop using iopl() in DPDK

Hi Andy,

On Thu, Oct 24, 2019 at 09:45:56PM -0700, Andy Lutomirski wrote:
> 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:
(...)

I'm just wondering, why wouldn't we introduce a sys_ioport() syscall
to perform I/Os in the kernel without having to play at all with iopl()/
ioperm() ? That would alleviate the need for these large port maps.
Applications that use outb/inb() usually don't need extreme speeds.
Each time I had to use them, it was to access a watchdog, a sensor, a
fan, control a front panel LED, or read/write to NVRAM. Some userland
drivers possibly don't need much more, and very likely run with
privileges turned on all the time, so replacing their inb()/outb() calls
would mostly be a matter of redefining them using a macro to use the
syscall instead.

I'd see an API more or less like this :

  int ioport(int op, u16 port, long val, long *ret);

<op> would take values such as INB,INW,INL to fill *<ret>, OUTB,OUTW,OUL
to read from <val>, possibly ORB,ORW,ORL to read, or with <val>, write
back and return previous value to <ret>, ANDB/W/L, XORB/W/L to do the
same with and/xor, and maybe a TEST operation to just validate support
at start time and replace ioperm/iopl so that subsequent calls do not
need to check for errors. Applications could then replace :

    ioperm() with ioport(TEST,port,0,0)
    iopl() with ioport(TEST,0,0,0)
    outb() with ioport(OUTB,port,val,0)
    inb() with ({ char val;ioport(INB,port,0,&val);val;})

... and so on.

And then ioperm/iopl can easily be dropped.

Maybe I'm overlooking something ?
Willy