Re: [dpdk-dev] [PATCH v5 0/3] support both PIO and MMIO BAR for virtio PMD

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 1/21/21 5:12 AM, 谢华伟(此时此刻） wrote:
> 
> On 2021/1/13 1:37, Maxime Coquelin wrote:
>>
>> On 10/22/20 5:51 PM, 谢华伟(此时此刻） wrote:
>>> From: "huawei.xhw" <huawei.xhw@alibaba-inc.com>
>>>
>>> Legacy virtio-pci only supports PIO BAR resource. As we need to
>>> create lots of
>>> virtio devices and PIO resource on x86 is very limited, we expose
>>> MMIO BAR.
>>>
>>> Kernel supports both PIO  and MMIO BAR for legacy virtio-pci device.
>>> We handles
>>> different type of BAR in the similar way.
>>>
>>> In previous implementation, with igb_uio we get PIO address from igb_uio
>>> sysfs entry; with uio_pci_generic, we get PIO address from
>>> /proc/ioports.
>>> For PIO/MMIO RW, there is different path for different drivers and arch.
>>> For VFIO, PIO/MMIO RW is through syscall, which has big performance
>>> issue.
>> Regarding the performance issue, do you have some numbers to share?
>> AFAICS, it can only have an impact on performance when interrupt mode is
>> used or queue notification is enabled.
>>
>> Does your HW Virtio implementation requires notification?
> 
> Yes, hardware needs notification to tell which queue has more buffer.
> 
> vhost backend also needs notification when it is not running in polling
> mode.
> 
> It is easy for software backend to sync with frontend whether it needs
> notification through memory but a big burden for hardware.

Yes, I understand, thanks for the clarification.

> Anyway, using vfio ioctl isn't needed at all. virtio PMD is only the
> consumer of pci_vfio_ioport_read.

My understanding is that using VFIO read/write ops is required for IOMMU
enabled case without cap_sys_rawio. And anyway, using inb/outb is just
bypassing VFIO. As I suggest in my other reply, it is better to document
that in the case of devices having PIO BARs, the user should consider
using UIO driver if performance is a concern.

> we could consider if we still need pci_vfio_ioport_read related API in
> future.

I disagree. I think the pci_vfio_ioport_* API is required at least for
the IOMMU enabled case.

Documentation is the way to go in my opinion, we can also add a warning
that performance may be degraded compared to UIO in
pci_vfio_ioport_map() when IOMMU is disabled if you think it may help
the users.

Thanks,
Maxime

> /huawei
>>
>> Is performance the only issue to have your HW working with Virtio PMD,
>> or is this series also fixing some functionnal issues?
>>
>> Best regards,
>> Maxime
>>
>