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

[Andy Lutomirski <luto@kernel.org>]

> On Oct 28, 2019, at 10:43 AM, Stephen Hemminger <stephen@networkplumber.org> wrote:
>
> On Fri, 25 Oct 2019 08:42:25 +0200
> Willy Tarreau <w@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.

If you are getting 10 MPPS with an OUT per packet, I’ll buy you a
whole case of beer.

I’m suggesting that, on virtio-legacy, you benchmark the performance
hit of using a syscall to ring the doorbell.  Right now, you're doing
an OUT instruction that traps to the hypervisor, probably gets
emulated, and goes out to whatever host-side driver is running.  The
cost of doing that is going to be quite high, especially on older
machines.  I'm guessing that adding a syscall to the mix won't make
much difference.

--Andy