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Re: [Qemu-devel] [PATCH v3] virtio-pci: fix migration for pci bus master
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v3] virtio-pci: fix migration for pci bus master |
Date: |
Tue, 21 Oct 2014 13:16:50 +0300 |
On Mon, Oct 20, 2014 at 06:08:56PM +0200, Alexander Graf wrote:
>
>
> On 20.10.14 08:58, Michael S. Tsirkin wrote:
> > Current support for bus master (clearing OK bit) together with the need to
> > support guests which do not enable PCI bus mastering, leads to extra state
> > in
> > VIRTIO_PCI_FLAG_BUS_MASTER_BUG bit, which isn't robust in case of
> > cross-version
> > migration for the case when guests use the device before setting DRIVER_OK.
> >
> > Rip out this code, and replace it:
> > - Modern QEMU doesn't need VIRTIO_PCI_FLAG_BUS_MASTER_BUG
> > so just drop it for latest machine type.
> > - For compat machine types, set PCI_COMMAND if DRIVER_OK
> > is set.
> >
> > As this is needed for 2.1 for both pc and ppc, move PC_COMPAT macros from
> > pc.h
> > to a new common header.
> >
> > Reviewed-by: Greg Kurz <address@hidden>
> > Tested-by: Greg Kurz <address@hidden>
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> >
> > Alexander, could you pls ack me merging this?
> > Thanks!
>
> Have you tried whether this works with old kernel versions? We
> introduced the broken flag for very specific old kernel versions that
> got their flags wrong IIRC.
>
>
> Alex
Yes. The point is that since 2.1 the broken flag isn't effective
anymore: when bus master is off, PCI blocks outgoing transactions and
setting internal flags isn't going to help.
Instead, we detect a buggy guest and immediately enable bus master for
it - no need for a separate flag.
--
MST