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From:   Long Li <longli@microsoft.com>
To:     Greg KH <gregkh@linuxfoundation.org>,
        Saurabh Sengar <ssengar@linux.microsoft.com>
CC:     Saurabh Singh Sengar <ssengar@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        Dexuan Cui <decui@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Michael Kelley (LINUX)" <mikelley@microsoft.com>
Subject: RE: [PATCH] uio_hv_generic: Enable interrupt for low speed VMBus
 devices
Thread-Topic: [PATCH] uio_hv_generic: Enable interrupt for low speed VMBus
 devices
Thread-Index: AQHY3zG2NgbGNaLQJkmnL/cmXn/Bqq4NgIeAgAYwnHA=
Date:   Tue, 18 Oct 2022 06:31:16 +0000
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> Subject: Re: [PATCH] uio_hv_generic: Enable interrupt for low speed VMBus
> devices
>=20
> On Thu, Oct 13, 2022 at 11:29:14AM -0700, Saurabh Sengar wrote:
> > Hyper-V is adding some "specialty" synthetic devices.
>=20
> What devices are those specifically?
>=20
> > Instead of writing new kernel-level VMBus drivers for these devices,
> > the devices will be presented to user space via this existing Hyper-V
> > generic UIO driver, so that a user space driver can handle the device.
> > Since these new synthetic devices are low speed devices, they don't
> > support monitor bits and we must use vmbus_setevent() to enable
> > interrupts from the host.
>=20
> That is not what the UIO interface is for.  Please write real drivers so =
that
> they tie into the specific user/kernel apis for those device types.
>=20
> Without a specific list of what these devices are, I can not recommend th=
at
> anyone use the UIO api for them as that's probably not a good idea.

There are some VMBUS drivers currently not implemented in Linux. Out of all
VMBUS drivers, two use "monitored bits": they are network and storage drive=
rs.
All the rest VMBUS drivers use hypercall for host notification and signal f=
or next
interrupt. One example of such driver is to collect process level crash inf=
ormation
for diagnostic purposes.

Also, we want to move some existing kernel mode VMBUS drivers to user-space=
,
such as hv_kvp and hv_filecopy. They don't really fit into an existing kern=
el API, and
they create their own devices under /dev and communicates with a user-mode
daemon to do most of the work. It's a better model that we can move those d=
rivers
entirely into user-mode.

>=20
> Also, if you do do this, you need to list where the source for that users=
pace
> code is so that users can get it and have their distros package it up for=
 them.  I
> do not see that here at all.
>=20
>=20
> >
> > Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> > ---
> >  drivers/uio/uio_hv_generic.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/uio/uio_hv_generic.c
> > b/drivers/uio/uio_hv_generic.c index c08a6cfd119f..8e5aa4a1247f 100644
> > --- a/drivers/uio/uio_hv_generic.c
> > +++ b/drivers/uio/uio_hv_generic.c
> > @@ -84,6 +84,9 @@ hv_uio_irqcontrol(struct uio_info *info, s32 irq_stat=
e)
> >  	dev->channel->inbound.ring_buffer->interrupt_mask =3D !irq_state;
> >  	virt_mb();
> >
> > +	if (!dev->channel->offermsg.monitor_allocated && irq_state)
> > +		vmbus_setevent(dev->channel);
> > +
> >  	return 0;
> >  }
> >
> > @@ -239,12 +242,6 @@ hv_uio_probe(struct hv_device *dev,
> >  	void *ring_buffer;
> >  	int ret;
> >
> > -	/* Communicating with host has to be via shared memory not
> hypercall */
> > -	if (!channel->offermsg.monitor_allocated) {
> > -		dev_err(&dev->device, "vmbus channel requires
> hypercall\n");
>=20
> I do not understand, why is this check not made anymore here?  Why
> constantly make the call above in the irq handler instead?  Isn't that go=
ing to
> be massively slow?

Some VMBUS devices exposed by the Hyper-V are not modeled as high speed,=20
they use hypercall, not monitored bits. Because they don't fit into other k=
ernel
API (as explained above), can we use UIO for those devices?

Thanks,
Long