[dpdk-dev] Please stop using iopl() in DPDK

[Stephen Hemminger]

On Fri, 25 Oct 2019 08:42:25 +0200
Willy Tarreau <w at 1wt.eu> wrote:

> 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

DPDK does not want to system calls. It kills performance.
With pure user mode access it can reach > 10 Million Packets/sec
with a system call per packet that drops to 1 Million Packets/sec.

Also, adding new system calls might help in the long term,
but users are often kernels that are at least 5 years behind
upstream.