Re: [Qemu-devel] [Xen-devel] [v2][PATCH 6/8] xen, gfx passthrough: support Intel IGD passthrough with VT-D

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, May 20, 2014 at 05:13:45AM +0000, Chen, Tiejun wrote:
> > -----Original Message-----
> > From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@oracle.com]
> > Sent: Monday, May 19, 2014 9:34 PM
> > To: Zhang, Yang Z
> > Cc: Chen, Tiejun; anthony.perard@citrix.com; stefano.stabellini@eu.citrix.com;
> > mst@redhat.com; Kelly.Zytaruk@amd.com; peter.maydell@linaro.org;
> > xen-devel@lists.xensource.com; weidong.han@intel.com; Kay, Allen M;
> > qemu-devel@nongnu.org; jean.guyader@eu.citrix.com;
> > anthony@codemonkey.ws
> > Subject: Re: [Xen-devel] [v2][PATCH 6/8] xen, gfx passthrough: support Intel
> > IGD passthrough with VT-D
> > 
> > On Mon, May 19, 2014 at 12:58:50AM +0000, Zhang, Yang Z wrote:
> > > Konrad Rzeszutek Wilk wrote on 2014-05-16:
> > > > On Fri, May 16, 2014 at 06:53:42PM +0800, Tiejun Chen wrote:
> > > > > Some registers of Intel IGD are mapped in host bridge, so it needs
> > > > > to passthrough these registers of physical host bridge to guest
> > > > > because emulated host bridge in guest doesn't have these mappings.
> > >
> > > Thanks for your review for the whole series patch.
> > 
> > Sure thing!
> > 
> > .. snip..
> > > > > +write_default:
> > > > > +    pci_default_write_config(pci_dev, config_addr, val, len);
> > > >
> > > >
> > > > and we just allow it through. But what happens if the guest decides
> > > > to change the BAR sizes?  Or fiddle with the GTT?
> > > >
> > > > Ouch. That really looks dangerous - or maybe I am too paranoid?
> > > >
> > >
> > > I do not quite understand your concern. We only pass through PAVPC to
> > physical host bridge. The others are handled by current logic. We don't change
> > any of it. So what problem will be exposed by this patch?
> > 
> > Ah, I assumed that pci_default_write_config would be writting everything
> > without any checks. But it looks to be doing the right thing and just sets the
> > emulated values.
> > 
> > Could we just add comment saying that it writes to the emulated values? That
> > way it won't trip folks.
> 
> diff --git a/hw/xen/xen_pt_graphics.c b/hw/xen/xen_pt_graphics.c
> index 1342f4f..5e5e3d5 100644
> --- a/hw/xen/xen_pt_graphics.c
> +++ b/hw/xen/xen_pt_graphics.c
> @@ -367,6 +367,7 @@ void igd_pci_write(PCIDevice *pci_dev, uint32_t config_addr,
>      case 0x58:      /* PAVPC Offset */
>          break;
>      default:
> +        /* Just sets the emulated values. */
>          goto write_default;
>      }
>  
> @@ -427,6 +428,7 @@ uint32_t igd_pci_read(PCIDevice *pci_dev, uint32_t config_addr, int len)
>      case 0xa8:        /* SNB: base of GTT stolen memory */
>          break;
>      default:
> +        /* Just gets the emulated values. */
>          goto read_default;

Thanks!
>      }
> 
> Thanks
> Tiejun