On Thu, Nov 11, 2021 at 12:41:02PM +0800, Jason Wang wrote: > > 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 > > Hi, > > 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)
Interesting point. Posted interrupts aren't supported by vhost-user's slave channel message. Since configuration change notifications are rare it's probably not a performance problem, but still. This makes me think vhost-user's approach is sub-optimal, it should have been an eventfd :(. Maybe the idea was that a slave message is less complex than adding an additional interface to set a configuration change notification eventfd. Let's not worry about it too much. I guess in the long run vhost-user can be rebased on top of the vDPA kernel interface (I wrote about that here: https://blog.vmsplice.net/2020/09/on-unifying-vhost-user-and-virtio.html). Stefan
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