Re: [Qemu-devel] [v4][PATCH 2/5] xen, gfx passthrough: create intel isa bridge

[Paolo Bonzini <pbonzini@redhat.com>]

Il 30/05/2014 10:59, Tiejun Chen ha scritto:
> +static int create_pch_isa_bridge(PCIBus *bus, XenHostPCIDevice *hdev)
> +{
> +    struct PCIDevice *dev;
> +
> +    char rid;
> +
> +    dev = pci_create(bus, PCI_DEVFN(0x1f, 0), "intel-pch-isa-bridge");

This is really a huge hack.  You're going to have two ISA bridge devices 
in the machine, with the BIOS imagining that the "good" one is at 1f.0 
and the ACPI tables actually describing the other one.  But the PCI 
device at 1f.0 remains there and a driver in the OS could catch it---not 
just intel_detect_pch---and if you want to add such a hack it should be 
done in the Xen management layers.

If possible, the host bridge patches are even worse.  If you change the 
vendor and device ID while keeping the registers of the i440FX you're 
going to get conflicts or break firmware badly; TianoCore and SeaBIOS 
both expect the normal i440FX vendor and device IDs, for example.

The hardcoded list of offsets is also not acceptable.  It is also not 
clear who is accessing the registers, whether the BIOS or the driver. 
For Linux, a cursory look at the driver shows that it only accesses 
0x50/0x52 of the listed offsets, but also 0x44/0x48 ("MCH BAR"), what 
happens if that code path is encountered?

The main problem with IGD passthrough is the incestuous (and that's a 
euphemism) relationship between the MCH, PCH and graphics driver.  It 
may make sense at the hardware level, but for virtualization it doesn't. 
  A virt-specific driver for GPU command passthrough (with aid from the 
kernel driver, but abstracting all the MCH/PCH-dependent details) would 
make much more sense.

It's really not your fault, there's not much you can do given the 
hardware architecture.  But I don't think this code can be accepted 
upstream, sorry.

Paolo