Re: [Qemu-devel] [PATCH] hw/pci: fixed crash when using rombar=0 for hotplugged devices

[Marcel Apfelbaum <marcel.a@redhat.com>]

On Wed, 2014-10-22 at 10:26 -0600, Alex Williamson wrote:
> On Wed, 2014-10-22 at 11:02 +0300, Michael S. Tsirkin wrote:
> > On Wed, Oct 22, 2014 at 10:41:05AM +0300, Marcel Apfelbaum wrote:
> > > On Wed, 2014-10-22 at 00:06 +0200, Paolo Bonzini wrote:
> > > > 
> > > > On 10/21/2014 02:37 PM, Marcel Apfelbaum wrote:
> > > > > ROM images must be loaded at startup. Usage of rombar=0 after that
> > > > > is not allowed, but should not crash QEMU.
> > > > > 
> > > > > Check that the device is not hotplugged before trying to
> > > > > insert the rom file.
> > > > 
> > > > I think it could also make sense to just ignore the option ROM and allow
> > > > the hotplug.
> > > We need a way to inform the user we did that, he *specifically* asked
> > > for a ROM he might need it.
> > > 
> > > Thanks,
> > > Marcel
> > 
> > But he also asked to disable BAR.
> > 
> > I don't see valid reasons for this configuration
> > expcept compatibility.
> > 
> > But I have a vague memory Alex thought differently.
> > Alex?
> 
> The comment in the original patch is really confusing for device
> assignment where rombar=0 is perfectly valid for any case, hotplug or
> not,
Yes, I should have made it more clear:
ROM images must be loaded at startup. Assigning ROM images
to devices and disabling the BAR at hotplug
is not allowed, but should not crash QEMU.


>  but only in combination with romfile= do we end up trying to use
> fw_cfg, which I think is what we're trying to prevent here.  Emulated
> devices are the ones that will still try to use fw_cfg because they have
> an implicit romfile.
> 
> I can't think of any use cases for requiring fw_cfg for an assigned
> device, it usually ends up being a user error to specify both rombar=0
> with romfile=$FILE.  Doing that for any device, emulated or assigned,
> disassociates the ROM from the device which breaks things like bootindex
> as well.
> 
> If a user specifies rombar=0,romfile=$FILE we should probably error and
> reject the device for hotplug.
>   Emulated devices with implicit romfiles
> are a bit harder to know what will break.  Silently dropping the
> implicit romfile seems like a reasonable thing,
It seems that this is the general agreement, I am going to post
a v2 for this and see what libvirt guys have to say about it.
I'll cc Eric.

Thanks,
Marcel

>  but then we have
> different behavior between cold- and hot-plugged devices.  I think
> that's reproducible for migration using romfile="", but I don't expect
> libvirt handles that properly.  It's a can of worms...
> 
> Thanks,
> Alex
> 
> > > > 
> > > > Sooner or later we should drop the oldest compat machine types...
> > > > everything until 0.12 probably could go.
> > > > 
> > > > Paolo
> > > > 
> > > > > Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
> > > > > ---
> > > > >  hw/pci/pci.c | 11 ++++++++++-
> > > > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > > > > index 6ce75aa..3907c90 100644
> > > > > --- a/hw/pci/pci.c
> > > > > +++ b/hw/pci/pci.c
> > > > > @@ -1776,7 +1776,12 @@ static int pci_qdev_init(DeviceState *qdev)
> > > > >          pci_dev->romfile = g_strdup(pc->romfile);
> > > > >          is_default_rom = true;
> > > > >      }
> > > > > -    pci_add_option_rom(pci_dev, is_default_rom);
> > > > > +
> > > > > +    rc = pci_add_option_rom(pci_dev, is_default_rom);
> > > > > +    if (rc != 0) {
> > > > > +        pci_unregister_device(DEVICE(pci_dev));
> > > > > +        return rc;
> > > > > +    }
> > > > >  
> > > > >      return 0;
> > > > >  }
> > > > > @@ -1940,6 +1945,10 @@ static int pci_add_option_rom(PCIDevice *pdev, bool is_default_rom)
> > > > >          if (class == 0x0300) {
> > > > >              rom_add_vga(pdev->romfile);
> > > > >          } else {
> > > > > +            if (DEVICE(pdev)->hotplugged) {
> > > > > +                error_report("PCI: rombar can't be 0 for hotplugged devices!");
> > > > > +                return -1;
> > > > > +            }
> > > > >              rom_add_option(pdev->romfile, -1);
> > > > >          }
> > > > >          return 0;
> > > > > 
> > > 
> > > 
> > 
> 
> 
>