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

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

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


> > 
> > Regarding the second, fixing IGD hardware to not rely on chipset magic is a
> > no-go, I agree.  I disagree that it's a no-go to define a "backdoor" that
> > lets a hypervisor pass the right information to the driver without hacking
> > the chipset device model.
> > 
> > The hardware folks would have to give us a place for a pair of registers
> > (something like data/address), and a bit somewhere else that would be always
> > 0 on hardware and always 1 if the hypervisor is implementing the pair of
> > registers.  This is similar to CPUID, which has the HYPERVISOR bit +
> > hypervisor-defined leaves at 0x40000000.
> > 
> > The data/address pair could be in a BAR, in configuration space, in the low
> > VGA ports at 0x3c0-0x3df, wherever.  The hypervisor bit can be in the same
> > place or somewhere else---again, whatever is convenient for the hardware
> > folks.  We just need *one bit* that is known-zero on all hardware, and 8
> > bytes in a reserved area.  I don't think it's too hard to find this space,
> > and I really, really would like Intel to follow up on a paravirtualized
> > backdoor.
> > 
> > That said, we have the problem of existing guests, so I agree something else
> > is needed.
> > 
> > >     a) Two bridges - one 'passthrough' and the legacy ISA bridge
> > >        that QEMU emulates. Both Linux and Windows are OK with
> > >        two bridges (even thought it is pretty weird).
> > 
> > This is pretty much the only solution for existing Linux guests that look up
> > the southbridge by class.
> 
> Right.
> > 
> > The proposed solution here is to define a new "pci stub" device in QEMU that
> > lets you define a do-nothing device with your desired vendor ID, device ID,
> > class and optionally subsystem IDs.
> 
> <nods>
> > 
> > The new machine type (the one that instantiates the special
> > IGD-passthrough-enabled northbridge) can then instantiate this stub device
> > at 1f.0 with the desired vendor ID, device ID and class ID.
> 
> Which is kind of neat because you can use a different type of device ID with 
> (say make it look like Ibex Peak) and pair it up with an IGD that is found
> only on LynxPoint. Oh fun!
> > 
> > If we cannot get the paravirtualized backdoor, it would also make sense to:
> > 
> > - have drivers standardize on a single way to probe the southbridge
> > 
> > - make this be neither by class (because the firmware wants to distinguish
> > the actual ISA bridge from the stub, and it can do so by looking up the
> > class), nor by slot (because this conflicts with the Q35 chipset model that
> > has the southbridge at 1f.0).
> > 
> > mst's proposal was to probe by subsystem id.  I'm not sure I understood the
> > details exactly, but I trust him. :)  However, in case it wasn't clear I
> > think a paravirtualized backdoor would still be better.
> 
> OK, like this:
> 
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index 651e65e..03f2829 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -433,6 +433,8 @@ void intel_detect_pch(struct drm_device *dev)
>                       unsigned short id = pch->device & 
> INTEL_PCH_DEVICE_ID_MASK;
>                       dev_priv->pch_id = id;
>  
> +                     if (pch->subsystem_vendor == PCI_VENDOR_ID_XEN)
> +                             id = pch->device & INTEL_PCH_DEVICE_ID_MASK;
>                       if (id == INTEL_PCH_IBX_DEVICE_ID_TYPE) {
>                               dev_priv->pch_type = PCH_IBX;
>                               DRM_DEBUG_KMS("Found Ibex Peak PCH\n");
> > 
> > >     b) One bridge - the one that QEMU emulates - and lets emulate
> > >        more of the registers (by emulate - I mean for some get the
> > >        data from the real hardware).
> > >
> > >           b1). We can't use the legacy because the registers are
> > >                above 256 (is that correct? Did I miss something?)
> > 
> > As I understand it, mst brought up Q35 because the northbridge configuration
> > space layout might be more similar to what the driver expects than for
> > PIIX4.  But I don't think anyone really said whether this is true or false.
> > 
> > I think Q35 is absolutely not a requirement for IGD passthrough, especially
> > until this statement is either proved or disproved.
> 
> OK, so lets drop that.
> > 
> > >4). Code does a bit of sysfs that could use some refacturing with
> > >    the KVM code.
> > >    Problem: More time needed to do the code restructing.
> > 
> > FWIW, I don't really care about code sharing with KVM.  That's a separate
> > problem and it's not necessary to bring it up and make waters even more
> > muddy.
> > 
> 
> OK, lets drop that for now.
> > Paolo

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