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Date:   Tue, 11 Apr 2023 02:55:32 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Angus Chen <angus.chen@jaguarmicro.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] virtio_pci: Wait for legacy device to be reset
Message-ID: <20230411025335-mutt-send-email-mst@kernel.org>
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On Tue, Apr 11, 2023 at 06:49:03AM +0000, Angus Chen wrote:
> 
> 
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, April 11, 2023 2:47 PM
> > To: Angus Chen <angus.chen@jaguarmicro.com>
> > Cc: Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH] virtio_pci: Wait for legacy device to be reset
> > 
> > On Tue, Apr 11, 2023 at 06:36:39AM +0000, Angus Chen wrote:
> > > Hi.
> > >
> > > > -----Original Message-----
> > > > From: Jason Wang <jasowang@redhat.com>
> > > > Sent: Tuesday, April 11, 2023 1:24 PM
> > > > To: Angus Chen <angus.chen@jaguarmicro.com>
> > > > Cc: mst@redhat.com; virtualization@lists.linux-foundation.org;
> > > > linux-kernel@vger.kernel.org
> > > > Subject: Re: [PATCH] virtio_pci: Wait for legacy device to be reset
> > > >
> > > > On Tue, Apr 11, 2023 at 9:39 AM Angus Chen
> > <angus.chen@jaguarmicro.com>
> > > > wrote:
> > > > >
> > > > > We read the status of device after reset,
> > > > > It is not guaranteed that the device be reseted successfully.
> > > > > We can use a while loop to make sure that,like the modern device did.
> > > > > The spec is not request it ,but it work.
> > > >
> > > > The only concern is if it's too late to do this.
> > > >
> > > > Btw, any reason you want to have a legacy hardware implementation. It
> > > > will be very tricky to work correctly.
> > >   En,I found this in the real production environment some times about one
> > year ago.
> > > and I fix this out of tree.our virtio card had been sold about thousands .
> > >
> > >   Now,we created a new card, it support virtio 0.95,1.0,1.1 etc.
> > 
> > I am not 100% sure what does this mean.
> > So it's a transitional device then?
> Hi mst,it is a real card in the In cloud computing ,not transitional device.

yes, a real card. But you said it supports 0.95, 1.0 and 1.1 guests,
so this is what the virtio spec calls a transitional device, right?

	To simplify transition from these earlier draft interfaces,
	a device MAY implement:

	\begin{description}
	\item[Transitional Device]
		a device supporting both drivers conforming to this
		specification, and allowing legacy drivers.
	\end{description}

or did I misunderstand?


> > 
> > 
> > >   And we use this host vdpa+ legacy virtio in vm to hot migration,we found
> > that the
> > >   Legacy model often get the middle state value after reset and probe again.
> > >   The Soc is Simulated by fpga which is run slower than the host,so the same
> > bug
> > >   Is found more frequently when the host use the other kernel like ubuntu or
> > centos8.
> > >
> > >   So we hope we can fix this by upstream .
> > > >
> > > > Thanks
> > > >
> > > > >
> > > > > Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
> > > > > ---
> > > > >  drivers/virtio/virtio_pci_legacy.c | 4 +++-
> > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_pci_legacy.c
> > b/drivers/virtio/virtio_pci_legacy.c
> > > > > index 2257f1b3d8ae..f2d241563e4f 100644
> > > > > --- a/drivers/virtio/virtio_pci_legacy.c
> > > > > +++ b/drivers/virtio/virtio_pci_legacy.c
> > > > > @@ -14,6 +14,7 @@
> > > > >   *  Michael S. Tsirkin <mst@redhat.com>
> > > > >   */
> > > > >
> > > > > +#include <linux/delay.h>
> > > > >  #include "linux/virtio_pci_legacy.h"
> > > > >  #include "virtio_pci_common.h"
> > > > >
> > > > > @@ -97,7 +98,8 @@ static void vp_reset(struct virtio_device *vdev)
> > > > >         vp_legacy_set_status(&vp_dev->ldev, 0);
> > > > >         /* Flush out the status write, and flush in device writes,
> > > > >          * including MSi-X interrupts, if any. */
> > > > > -       vp_legacy_get_status(&vp_dev->ldev);
> > > > > +       while (vp_legacy_get_status(&vp_dev->ldev))
> > > > > +               msleep(1);
> > > > >         /* Flush pending VQ/configuration callbacks. */
> > > > >         vp_synchronize_vectors(vdev);
> > > > >  }
> > > > > --
> > > > > 2.25.1
> > > > >
> > >
>