On 2021/1/21 16:47, Maxime Coquelin wrote:

On 1/21/21 5:12 AM, 谢华伟(此时此刻） wrote:

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On 2021/1/13 1:37, Maxime Coquelin wrote:

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On 10/22/20 5:51 PM, 谢华伟(此时此刻） wrote:

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From: "huawei.xhw" <redacted>

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.

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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.

Get it. so user could read/write PIO using VFIO without iopl permission, 
with some performance penalty.

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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

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/huawei

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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