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

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Message-ID: <20250310173123.GA12960@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Mon, 10 Mar 2025 10:31:23 -0700
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Long Li <longli@...rosoft.com>
Cc: "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>,
	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

On Mon, Mar 10, 2025 at 05:16:15PM +0000, Long Li wrote:
> > 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.
> 

Thanks for the details. However I didn't understand if memory barrier is not
required why have it only for 'slow' devices (ie !monitor_allocated) ?

This change also removes the MB for primary channel (if it supports moitor bits),
if this is intentional, we should call out this in commit message.

Regards,
Saurabh