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Re: [Xen-devel] [PATCH] Fixing ATS enabling for device assignment

Jan Beulich wrote:
"Han, Weidong"  05/19/10 4:37 AM >>>
Currently, Xen only enables ATS in Xen booting. When a ATS capable device is 
assigned to guest, ATS is actually not enabled because FLR before assignment 
causes it to be disabled. Thus ATS cannot be used in guest. This patch enables 
ATS in domain_context_mapping. This ensures ATS is enabled in assignment 
because FLR is earlier than domain_context_mapping call. Therefore ATS can be 
used in guest. This patch also implements disable_ats_device to disable ATS 
when the device is deassigned from a domain.

Hmm, this looks inconsistent to me: Iirc FLR is being done from the tools, so 
unless it is technically impossible to do FLR after domain_context_mapping(), 
the patch creates a hypervisor dependency on the tools (if nothing else, a 
latent bug - to be introduced the moment operation ordering changes in the 
It doesn't depend on FLR from tools. It just ensures ATS is enabled for device assignment in guest. If ATS is already enabled, enable_ats_device won't re-enable it.
Further, if ATS gets enabled at boot time, why would you want to disable it 
after de-assignment?
ATS enabled devices will be added to ats_devices list. disabling ATS will remove a device from that list and avoid leak. If ATS is enabled on a VF, and it's destroyed by removing PF driver, disable_ats_device will be invoked to remove it from the list.

Finally (only partially related) - why are {en,dis}able_ats_device() 
architecture specific? PCI config space accesses really aren't I would think

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