linux-kernel - Re: [PATCH v2] virtio: only reset device and restore status if needed in device resume

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Message-ID: <20241106042828-mutt-send-email-mst@kernel.org>
Date: Wed, 6 Nov 2024 04:28:53 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: qiang4.zhang@...ux.intel.com, Paolo Bonzini <pbonzini@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
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	Herbert Xu <herbert@...dor.apana.org.au>,
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	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Gonglei <arei.gonglei@...wei.com>,
	"David S. Miller" <davem@...emloft.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Chen, Jian Jun" <jian.jun.chen@...el.com>,
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	Eric Dumazet <edumazet@...gle.com>,
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Subject: Re: [PATCH v2] virtio: only reset device and restore status if
 needed in device resume

On Fri, Nov 01, 2024 at 10:11:11AM +0800, Jason Wang wrote:
> On Fri, Nov 1, 2024 at 9:54 AM <qiang4.zhang@...ux.intel.com> wrote:
> >
> > From: Qiang Zhang <qiang4.zhang@...el.com>
> >
> > Virtio core unconditionally reset and restore status for all virtio
> > devices before calling restore method. This breaks some virtio drivers
> > which don't need to do anything in suspend and resume because they
> > just want to keep device state retained.
> 
> The challenge is how can driver know device doesn't need rest.

I actually don't remember why do we do reset on restore. Do you?


> For example, PCI has no_soft_reset which has been done in the commit
> "virtio: Add support for no-reset virtio PCI PM".
> 
> And there's a ongoing long discussion of adding suspend support in the
> virtio spec, then driver know it's safe to suspend/resume without
> reset.
> 
> >
> > Virtio GPIO is a typical example. GPIO states should be kept unchanged
> > after suspend and resume (e.g. output pins keep driving the output) and
> > Virtio GPIO driver does nothing in freeze and restore methods. But the
> > reset operation in virtio_device_restore breaks this.
> 
> Is this mandated by GPIO or virtio spec? If yes, let's quote the revelant part.
> 
> >
> > Since some devices need reset in suspend and resume while some needn't,
> > create a new helper function for the original reset and status restore
> > logic so that virtio drivers can invoke it in their restore method
> > if necessary.
> 
> How are those drivers classified?
> 
> >
> > Signed-off-by: Qiang Zhang <qiang4.zhang@...el.com>
> 
> Thanks