Re: [Xen-devel] [PATCH][VTD] enabling PCI ACS P2P upstream forwarding

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Nov 17, 2009 at 01:49:09PM -0800, Kay, Allen M wrote:
> This patch enables P2P upstream forwarding in ACS capable PCIe switches.  The 
> enabling is conditioned on iommu_enabled variable.  This code solves two 
> potential problems in virtualization environment where a PCIe device is 
> assigned to a guest domain using a HW iommu such as VT-d:
> 
> 1) Unintentional failure caused by guest physical address programmed into the 
> device's DMA that happens to match the memory address range of other 
> downstream ports in the same PCIe switch.  This causes the PCI transaction to 
> go to the matching downstream port instead of go to the root complex to get 
> translated by VT-d as it should be.
> 
> 2) Malicious guest software intentionally attacks another downstream PCIe 
> device by programming the DMA address into the assigned device that matches 
> memory address range of the downstream PCIe port.
> 
> Corresponding ACS filtering code is already in upstream control panel code 
> that do not allow PCI device passthrough to guests if it is behind a PCIe 
> switch that does not have ACS capability or with ACS capability but is not 
> enabled. 

Based on your description it sounds like the function should be called: 
pci_reset_acs.
Should there be a corresponding function to disable the P2P upstream forwarding?

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