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Re: [PATCH v10 00/10]vhost-vdpa: add support for configure interrupt

From: Jason Wang
Subject: Re: [PATCH v10 00/10]vhost-vdpa: add support for configure interrupt
Date: Thu, 11 Nov 2021 12:41:02 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.14.0

在 2021/11/8 下午6:53, Stefan Hajnoczi 写道:
On Fri, Nov 05, 2021 at 12:48:17AM +0800, Cindy Lu wrote:
these patches add the support for configure interrupt

These codes are all tested in vp-vdpa (support configure interrupt)
vdpa_sim (not support configure interrupt), virtio tap device

test in virtio-pci bus and virtio-mmio bus
vhost-user has a configuration space change notification but it uses a
slave channel message (VHOST_USER_SLAVE_CONFIG_CHANGE_MSG) instead of an
eventfd. Ideally the vhost kernel ioctl and vhost-user interfaces would
follow the same design.

I'm concerned "common" vhost code is going to end up with lots of
callbacks that are not available uniformly across vhost kernel, vdpa,
and vhost-user. That makes it hard to understand and debug vhost, plus
differences make it harder to to correctly extend these interfaces in
the future.

Is the decision to a new eventfd-based interface instead of
vhost_chr_read/write() deliberate?

I think this is a good question. Here're some reasons for using eventfd from the kernel perspective:

1) the eventfd is used for relaying interrupts for vqs, so we choose to use that for the config interrupt
2) make it possible to be used for irq bypassing (posted interrupt)

From Qemu's perspective, as shown in this series, it looks more easier to be integrated with the both vhost and transport layer (e.g vector masking etc).


If yes, do you think VHOST_USER_SLAVE_CONFIG_CHANGE_MSG was a design
mistake in vhost-user that needs to be fixed?


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