linux-kernel - Re: [PATCH 0/6] Low speed Hyper-V devices support

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Date: Sat, 17 Feb 2024 23:51:14 -0800
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, ssengar@...rosoft.com
Subject: Re: [PATCH 0/6] Low speed Hyper-V devices support

On Sun, Feb 18, 2024 at 08:10:36AM +0100, Greg KH wrote:
> On Sat, Feb 17, 2024 at 10:03:34AM -0800, Saurabh Sengar wrote:
> > Hyper-V is adding multiple low speed "speciality" synthetic devices.
> > Instead of writing a new kernel-level VMBus driver for each device,
> > make the devices accessible to user space through UIO-based
> > uio_hv_generic driver. Each device can then be supported by a user
> > space driver. This approach optimizes the development process and
> > provides flexibility to user space applications to control the key
> > interactions with the VMBus ring buffer.
> > 
> > The new synthetic devices are low speed devices that don't support
> > VMBus monitor bits, and so they must use vmbus_setevent() to notify
> > the host of ring buffer updates. These new devices also have smaller
> > ring buffer sizes which requires to add support for variable ring buffer
> > sizes.
> > 
> > Moreover, this patch series adds a new implementation of the fcopy
> > application that uses the new UIO driver. The older fcopy driver and
> > application will be phased out gradually. Development of other similar
> > userspace drivers is still underway.
> > 
> > 
> > Efforts have been made previously to implement this solution earlier.
> > Here are the discussions related to those attempts:
> > https://lore.kernel.org/lkml/1691132996-11706-1-git-send-email-ssengar@linux.microsoft.com/
> > https://lore.kernel.org/lkml/1665575806-27990-1-git-send-email-ssengar@linux.microsoft.com/
> > https://lore.kernel.org/lkml/1665685754-13971-1-git-send-email-ssengar@linux.microsoft.com/
> 
> So is this a v4 of the patch series?  What has changed from those
> previous submissions?

In the most recent attempt we introduced a new driver uio_hv_vmbus_client
for slow devices, where as in this new approach we are making use of
existing uio_hv_generic driver.

We also introduced the function to query ring buffer sizes, this is an
attempt to address your comment on earlier series to avoid kernel params.
Comment ref: https://lore.kernel.org/lkml/Y0bipdisMbTNMYOq@kroah.com/

We later tried to have ring buffer sizes via sysfs for which we wrote a
new driver uio_hv_vmbus_client as explained above.

This is the next approach in an attempt to address all of the concerns
with all the previous series.

- Saurabh


> 
> thanks,
> 
> greg k-h