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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Thu, Jul 03, 2014 at 12:09:28PM -0700, Jesse Barnes wrote:
> On Thu, 3 Jul 2014 14:26:12 -0400
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> 
> > 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.
> 
> The display part of the GPU is partly on the PCH, and it's possible to
> mix & match them a bit, so we have this detection code to figure things
> out.  In some cases, the PCH display portion may not even be present,
> so we look for that too.
> 
> Practically speaking, we could probably assume specific CPU/PCH combos,
> as I don't think they're generally mixed across generations, though SNB
> and IVB did have compatible sockets, so there is the possibility of
> mixing CPT and PPT PCHs, but those are register identical as far as the
> graphics driver is concerned, so even that should be safe.
> 
> Beyond that, the other MCH data we need to look at is mirrored into the
> GPU's MMIO space on current gens.  On older gens, we do need to poke at
> the memory controller a bit to get some info (see
> intel_setup_mchbar()), but that's not true of newer stuff.  Looks like
> we only short circuit that on VLV though; we could probably do it on
> SNB+.

That sounds great. Tiejun could you confirm that with
windows driver guys too?

> -- 
> Jesse Barnes, Intel Open Source Technology Center

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