Re: [Xen-devel] GPU passthrough issue when VM is configured with 4G memory

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Mar 12, 2013 at 5:45 AM, Hanweidong <hanweidong@xxxxxxxxxx> wrote:
> I don't think it's a pass-through issue. We also encountered similar
> issue when using ivshmem device with 512M mmio. The problem is hvmloader
> adjusted pci_mem_start according to actual mmio size of VM, but QEMU does
> not know the actual mmio size of VM, it just uses the HVM_BELOW_4G_RAM_END
> and HVM_BELOW_4G_MMIO_LENGTH in xen_ram_init. This results in memory layout
> mismatch between hvmloader and QEMU. I think it needs to make QEMU know
> actual mmio size, and then it can make the same memory layout as hvmloader.

So I'm not an expert in this codepath, but it was my understanding
that resizing the MMIO hole is actually the job of the guest BIOS: it
is supposed to query the PCI devices to find out how much memory they
need, and then relocate memory as appropriate.  XenServer never had
any problem with the MMIO hole not being big enough when passing cards
to the guest; the only problem we've ever had (as far as I know) is
the issue I mentioned before, where there was a bug in some pci bridge
cards that breaks VT-d if the guest physical address overlaps the host
physical address range of another device.

Can you maybe boot Linux in an HVM domain and do an lspci or look at
Linux's e820 map, and check to see whether the BIOS has in fact
relocated the guest memory, or whether the MMIO hole does indeed start
only at HVM_BELOW_4G_RAM_END?

 -George

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