linux-kernel - RE: [Patch v2] uio_hv_generic: Set event for all channels on the device

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Message-ID:
 <SA6PR21MB4231D4A8F6D942B405777BECCED62@SA6PR21MB4231.namprd21.prod.outlook.com>
Date: Mon, 10 Mar 2025 17:16:15 +0000
From: Long Li <longli@...rosoft.com>
To: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
	"longli@...uxonhyperv.com" <longli@...uxonhyperv.com>
CC: KY Srinivasan <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [Patch v2] uio_hv_generic: Set event for all channels on the
 device

> Subject: Re: [Patch v2] uio_hv_generic: Set event for all channels on the device
> 
> On Fri, Feb 28, 2025 at 02:14:14PM -0800, longli@...uxonhyperv.com wrote:
> > From: Long Li <longli@...rosoft.com>
> >
> > Hyper-V may offer a non latency sensitive device with subchannels
> > without monitor bit enabled. The decision is entirely on the Hyper-V
> > host not configurable within guest.
> >
> > When a device has subchannels, also signal events for the subchannel
> > if its monitor bit is disabled.
> >
> > Signed-off-by: Long Li <longli@...rosoft.com>
> > ---
> > Change log
> > v2: Use vmbus_set_event() to avoid additional check on monitored bit
> >     Lock vmbus_connection.channel_mutex when going through subchannels
> >
> >  drivers/uio/uio_hv_generic.c | 32 ++++++++++++++++++++++++++------
> >  1 file changed, 26 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/uio/uio_hv_generic.c
> > b/drivers/uio/uio_hv_generic.c index 3976360d0096..45be2f8baade 100644
> > --- a/drivers/uio/uio_hv_generic.c
> > +++ b/drivers/uio/uio_hv_generic.c
> > @@ -65,6 +65,16 @@ struct hv_uio_private_data {
> >  	char	send_name[32];
> >  };
> >
> > +static void set_event(struct vmbus_channel *channel, s32 irq_state) {
> > +	channel->inbound.ring_buffer->interrupt_mask = !irq_state;
> > +	if (!channel->offermsg.monitor_allocated && irq_state) {
> > +		/* MB is needed for host to see the interrupt mask first */
> > +		virt_mb();
> 
> Why is memory barrier not getting called for 'faster' channels ?
> 
> - Saurabh

No, the memory barrier is not needed. Even with a barrier, There is no guarantee that all pending IRQs are flushed when hv_uio_irqcontrol() returns. If user-mode depends on this guarantee, that user-mode has a bug. This barrier adds unnecessary costs when walking through subchannels.

Long