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Re: [Qemu-devel] [PATCH v2] virtio-pci: enable bus master for old guests
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v2] virtio-pci: enable bus master for old guests |
Date: |
Thu, 11 Sep 2014 20:52:22 +0300 |
On Thu, Sep 11, 2014 at 06:00:39PM +0200, Greg Kurz wrote:
> On Wed, 10 Sep 2014 20:30:47 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
>
> > commit cc943c36faa192cd4b32af8fe5edb31894017d35
> > pci: Use bus master address space for delivering MSI/MSI-X messages
> > breaks virtio-net for rhel6.[56] x86 guests because they don't
> > enable bus mastering for virtio PCI devices
> >
> > Old guests forgot to enable bus mastering, enable it
> > automatically when driver discovers device.
> >
> > Cc: address@hidden
> > Reported-by: Greg Kurz <address@hidden>
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> >
> > OK, this should have better luck.
>
> Unfortunately not... it is even worse than V1 as
> both x86 and ppc64 guests fail. I've added some
> debug messages in virtio_ioport_write() and
> virtio_write_config() to track:
> - VIRTIO_PCI_STATUS changes
> - bus master disablement
>
> This is what I get for x86 with a virtio-net device:
>
> virtio_write_config: PCI_COMMAND = 0x3
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x0
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x1 ==> v2 patch enables BM
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x3
> virtio_write_config: PCI_COMMAND = 0x3 ==> guest disables BM
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x7
> virtio_ioport_write: detected bus master bug ==> v1 patch would have
> enabled BM here
>
> and for ppc64 when booting from a virtio-blk disk:
>
> virtio_write_config: PCI_COMMAND = 0x3
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x0
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x1 ==> v2 patch enables BM
> virtio_ioport_write: VIRTIO_PCI_STATUS = 0x3
> virtio_write_config: PCI_COMMAND = 0x3 ==> guest disables BM
So guest sets DRIVER then overwrites PCI_COMMAND. Weird.
Any idea which guest code does this?
> I think we should:
> - detect the "old driver" when reaching DRIVER as it works for
> both architectures
> - always re-enable BM when an "old driver" disables it
>
> That is basically your v1 plus its follow-up patch:
>
> https://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg01918.html
>
> plus:
>
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -317,7 +317,7 @@ static void virtio_ioport_write(void *opaque, uint32_t
> addr, uint32_t val)
> /* Linux before 2.6.34 sets the device as OK without enabling
> the PCI device bus master bit. In this case we need to disable
> some safety checks. */
> - if ((val & VIRTIO_CONFIG_S_DRIVER_OK) &&
> + if ((val & VIRTIO_CONFIG_S_DRIVER) &&
> !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
>
> memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
>
> I could verify it fixes the issues. I will send a patch shortly.
>
>
> Cheers.
>
> --
> Greg
>
> > This also makes it possible to simplify code,
> > will do that in a follow-up patch.
> >
> >
> > hw/virtio/virtio-pci.c | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index ddb5da1..a29d94f 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -303,6 +303,14 @@ static void virtio_ioport_write(void *opaque, uint32_t
> > addr, uint32_t val)
> > virtio_pci_stop_ioeventfd(proxy);
> > }
> >
> > + /* Linux before 2.6.34 uses the device without enabling
> > + the PCI device bus master bit. As a work-around, enable it
> > + automatically when driver detects the device. */
> > + if (val == VIRTIO_CONFIG_S_ACKNOWLEDGE) {
> > +
> > memory_region_set_enabled(&proxy->pci_dev.bus_master_enable_region,
> > + true);
> > + }
> > +
> > virtio_set_status(vdev, val & 0xFF);
> >
> > if (val & VIRTIO_CONFIG_S_DRIVER_OK) {