[Intel-gfx] ResettRe: [Xen-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

[Konrad Rzeszutek Wilk]

On Thu, Jul 03, 2014 at 10:32:12AM +0300, Michael S. Tsirkin wrote:
> On Wed, Jul 02, 2014 at 12:23:37PM -0400, Konrad Rzeszutek Wilk wrote:
> > On Wed, Jul 02, 2014 at 04:50:15PM +0200, Paolo Bonzini wrote:
> > > Il 02/07/2014 16:00, Konrad Rzeszutek Wilk ha scritto:
> > > >With this long thread I lost a bit context about the challenges
> > > >that exists. But let me try summarizing it here - which will hopefully
> > > >get some consensus.
> > > >
> > > >1). Fix IGD hardware to not use Southbridge magic addresses.
> > > >    We can moan and moan but I doubt it is going to change.
> > > 
> > > There are two problems:
> > > 
> > > - Northbridge (i.e. MCH i.e. PCI host bridge) configuration space addresses
> > 
> > Right. So in  drivers/gpu/drm/i915/i915_dma.c:
> > 1135 #define MCHBAR_I915 0x44                                                        
> > 1136 #define MCHBAR_I965 0x48                     
> > 
> > 1147         int reg = INTEL_INFO(dev)->gen >= 4 ? MCHBAR_I965 : MCHBAR_I915;        
> > 1152         if (INTEL_INFO(dev)->gen >= 4)                                          
> > 1153                 pci_read_config_dword(dev_priv->bridge_dev, reg + 4, &temp_hi); 
> > 1154         pci_read_config_dword(dev_priv->bridge_dev, reg, &temp_lo);             
> > 1155         mchbar_addr = ((u64)temp_hi << 32) | temp_lo;                
> > 
> > and
> > 
> > 1139 #define DEVEN_REG 0x54                                                          
> > 
> > 1193         int mchbar_reg = INTEL_INFO(dev)->gen >= 4 ? MCHBAR_I965 : MCHBAR_I915; 
> > 1202         if (IS_I915G(dev) || IS_I915GM(dev)) {                                  
> > 1203                 pci_read_config_dword(dev_priv->bridge_dev, DEVEN_REG, &temp);  
> > 1204                 enabled = !!(temp & DEVEN_MCHBAR_EN);                           
> > 1205         } else {                                                                
> > 1206                 pci_read_config_dword(dev_priv->bridge_dev, mchbar_reg, &temp); 
> > 1207                 enabled = temp & 1;                                             
> > 1208         }                                                
> > > 
> > > - Southbridge (i.e. PCH i.e. ISA bridge) vendor/device ID; some versions of
> > > the driver identify it by class, some versions identify it by slot (1f.0).
> > 
> > Right, So in  drivers/gpu/drm/i915/i915_drv.c the giant intel_detect_pch
> > which sets the pch_type based on :
> > 
> >  432                 if (pch->vendor == PCI_VENDOR_ID_INTEL) {                       
> >  433                         unsigned short id = pch->device & INTEL_PCH_DEVICE_ID_MASK;
> >  434                         dev_priv->pch_id = id;                                  
> >  435                                                                                 
> >  436                         if (id == INTEL_PCH_IBX_DEVICE_ID_TYPE) { 
> > 
> > It checks for 0x3b00, 0x1c00, 0x1e00, 0x8c00 and 0x9c00.
> > The INTEL_PCH_DEVICE_ID_MASK is 0xff00
> > > 
> > > To solve the first, make a new machine type, PIIX4-based, and pass through
> > > the registers you need.  The patch must document _exactly_ why the registers
> > > are safe to pass.  If they are not reserved on PIIX4, the patch must
> > > document what the same offsets mean on PIIX4, and why it's sensible to
> > > assume that firmware for virtual machine will not read/write them.  Bonus
> > > point for also documenting the same for Q35.
> > 
> > OK. They look to be related to setting up an MBAR , but I don't understand
> > why it is needed. Hopefully some of the i915 folks CC-ed here can answer.
> 
> In particular, I think setting up MBAR should move out of i915 and become
> the bridge driver tweak on linux and windows.

That is an excellent idea.

However I am still curious to _why_ it has to be done in the first place.

> It would then run on the priveledged host
> and we won't need to deal with it in QEMU.