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

[Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>]

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+.

-- 
Jesse Barnes, Intel Open Source Technology Center

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