Re: [Qemu-devel] Allocate PCI MMIO without BAR requests.

[Jan Kiszka]

Hi David,

On 2015-06-26 17:32, David kiarie wrote:
> Hi all,
> 
> Some efforts to emulate AMD IOMMU have being going over the past few months.
> 
> In real hardware AMD IOMMU is implemented as a PCI function. When
> emulating it in Qemu we want to allocate it MMIO space but real AMD
> IOMMU manage to reserve memory without making a BAR request, probably
> through a static address that's written by the device.(This is
> something similar to what non-PCI bus devices do).Trying to reserve
> memory via a BAR request results in address conflicts(in Linux) and
> all other PCI devices reserve platform resources via BAR requests.

The AMD IOMMU spec makes it even clearer:

"3 Registers

The IOMMU is configured and controlled via two sets of registers — one
in the PCI configuration space and another set mapped in system address
space. [...]

3.1 PCI Resources

[...] A PCI Function containing an IOMMU capability block does not
include PCI BAR registers."

> 
> I would like to hear suggestions on how to reserve a memory region for
> the device without making a BAR request.

I see two approaches:

 - Let the IOMMU sit on two buses, PCI and system, i.e. become a PCI
   and SysBus device at the same time - I suspect, though, that this
   cannot be modeled with QOM right now.

 - Model the MMIO registers via the BAR interface but overwrite the
   PCI config space so that no BAR becomes visible and make sure that
   writes to the PCI command register cannot disable this region (which
   would be the case with normal BARs). Hackish, but it seems feasible.

Jan

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