Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

[Santosh Shukla]

On Tue, Dec 29, 2015 at 11:01 PM, Alex Williamson
<alex.williamson@xxxxxxxxxx> wrote:
> On Tue, 2015-12-29 at 22:00 +0530, Santosh Shukla wrote:
>> On Tue, Dec 29, 2015 at 9:50 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> > On Tuesday 29 December 2015 21:25:15 Santosh Shukla wrote:
>> > > mistakenly added wrong email-id of alex, looping his correct one.
>> > >
>> > > On 29 December 2015 at 21:23, Santosh Shukla <santosh.shukla@lina
>> > > ro.org> wrote:
>> > > > On 29 December 2015 at 18:58, Arnd Bergmann <arnd@xxxxxxxx>
>> > > > wrote:
>> > > > > On Wednesday 23 December 2015 17:04:40 Santosh Shukla wrote:
>> > > > > > On 23 December 2015 at 03:26, Arnd Bergmann <arnd@xxxxxxxx>
>> > > > > > wrote:
>> > > > > > > On Tuesday 22 December 2015, Santosh Shukla wrote:
>> > > > > > > > }
>> > > > > > > >
>> > > > > > > > So I care for /dev/ioport types interface who could do
>> > > > > > > > more than byte
>> > > > > > > > data copy to/from user-space. I tested this patch with
>> > > > > > > > little
>> > > > > > > > modification and could able to run pmd driver for
>> > > > > > > > arm/arm64 case.
>> > > > > > > >
>> > > > > > > > Like to know how to address pci_io region mapping
>> > > > > > > > problem for
>> > > > > > > > arm/arm64, in-case /dev/ioports approach is not
>> > > > > > > > acceptable or else I
>> > > > > > > > can spent time on restructuring the patch?
>> > > > > > > >
>> > > > > > >
>> > > > > > > For the use case you describe, can't you use the vfio
>> > > > > > > framework to
>> > > > > > > access the PCI BARs?
>> > > > > > >
>> > > > > >
>> > > > > > I looked at file: drivers/vfio/pci/vfio_pci.c, func
>> > > > > > vfio_pci_map() and
>> > > > > > it look to me that it only maps ioresource_mem pci region,
>> > > > > > pasting
>> > > > > > code snap:
>> > > > > >
>> > > > > > if (!(pci_resource_flags(pdev, index) & IORESOURCE_MEM))
>> > > > > > return -EINVAL;
>> > > > > > ....
>> > > > > >
>> > > > > > and I want to map ioresource_io pci region for arm platform
>> > > > > > in my
>> > > > > > use-case. Not sure vfio maps pci_iobar region?
>> > > > >
>> > > > > Mapping I/O BARs is not portable, notably it doesn't work on
>> > > > > x86.
>> > > > >
>> > > > > You should be able access them using the read/write interface
>> > > > > on
>> > > > > the vfio device.
>> > > > >
>> > > > Right, x86 doesn't care as iopl() could give userspace
>> > > > application
>> > > > direct access to ioports.
>> > > >
>> > > > Also, Alex in other dpdk thread [1] suggested someone to
>> > > > propose io
>> > > > bar mapping in vfio-pci, I guess in particular to non-x86 arch
>> > > > so I
>> > > > started working on it.
>> > > >
>> > >
>> >
>> > So what's wrong with just using the existing read/write API on all
>> > architectures?
>> >
>>
>> nothing wrong, infact read/write api will still be used so to access
>> mmaped io pci bar at userspace. But right now vfio_pci_map() doesn't
>
> vfio_pci_mmap(), the read/write accessors fully support i/o port.
>

(Sorry for delayed response!)
Right.
>> map io pci bar in particular (i.e.. ioresource_io) so I guess need to
>> add that bar mapping in vfio. pl. correct me if i misunderstood
>> anything.
>
> Maybe I misunderstood what you were asking for, it seemed like you
> specifically wanted to be able to mmap i/o port space, which is
> possible, just not something we can do on x86.  Maybe I should have
> asked why.  The vfio API already supports read/write access to i/o port

Yes, I want to map io port pci space in vfio and reason for that is :
I want to access virto-net-pci device at userspace using vfio and for
that I am using vfio-noiommu latest linux-next patch. but I am not
able to mmap io port pci space in vfio because of below condition -

1)
--- user space code snippet ----
reg.index = i; // where i is {0..1} i.e.. {BAR0..BAR1} such that BAR0
= io port pci space and BAR1 = pci config space

ret = ioctl(vfio_dev_fd, VFIO_DEVICE_GET_REGION_INFO, &reg);
if ((reg.flags & VFIO_REGION_INFO_FLAG_MMAP) == 0) {
       return err;
}
now consider i = 0 case where pci_rersource_flag set to IORESOURCE_IO

--- kernel / vfip-pci.c -------------
so vfio_pci_ioctl() wont set info.flag to VFIO_REGION_INFO_FLAG_MMAP.
And it won't set for two
1) pci_resource_flag & IORESOURCE_MEM
2) ioport size < PAZE_SIZE

The second one I addressed but first one is what I believe that need
to add support in vfio.
and Same applicable for vfio_pci_mmap() too..

This is why I am thinking to add IORESOURCE_IO space mapping support
in vfio; in particular non-x86 archs.. pl. correct my understanding in
case wrong.

> space, so if you intend to mmap it only to use read/write on top of the
> mmap, I suppose you might see some performance improvement, but not
> really any new functionality.  You'd also need to deal with page size
> issues since i/o port ranges are generally quite a bit smaller than the
> host page size and they'd need to be mapped such that each devices does
> not share a host page of i/o port space with other devices.  On x86 i/o

Yes. I have taken care size < PAZE_SIZE condition.

> port space is mostly considered legacy and not a performance critical
> path for most modern devices; PCI SR-IOV specifically excludes i/o port
> space.  So what performance gains do you expect to see in being able to
> mmap i/o port space and what hardware are you dealing with that relies
> on i/o port space rather than mmio for performance?  Thanks,
>
dpdk user space virtio-net pmd driver uses ioport space for driver
initialization, as because virtio-net header resides in ioport area of
virtio-pxe.rom file, also it is inlined to virtio spec (<= 0.95). Till
now virtio-net dpdk pmd driver for x86 using iopl() to access those
ioport for driver initialization but for non-x86 cases; we needed
alternative i.e.. kernel to someway map ioport pci region either by
architecture example powerpc does Or look in vfio for mapping. I hope
I made my use-case clear.

> Alex
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