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

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Fixing ATS enabling for device assignment
From: Weidong Han <weidong.han@xxxxxxxxx>
Date: Wed, 19 May 2010 17:52:53 +0800
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "keir.fraser@xxxxxxxxxxxxx" <keir.fraser@xxxxxxxxxxxxx>
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Jan Beulich wrote:
Weidong Han  05/19/10 11:12 AM >>>
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.

Perhaps I didn't express my concern properly: It's not really the hypervisor 
depending on anything done by the tools, but the code you ad to the hypervisor 
will trun out pointless if the tools move FLR past the point where 
domain_context_mapping() gets executed.
Yes, it's a problem in your case. But FLR is required to be earlier than domain_context_mapping for device assignment.
 It would seem more clean to me if e.g. the tools re-enabled ATS after doing 
FLR if it was enabled before.I
Sounds good. But i'm not sure if it's suitable or easy to maintain capability enabling/disabling in control panel, I feel current device handling logic (hotplug, assign/deassign, PV and hvm) in control panel is a bit of complex.
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.

That's fine, but doesn't answer my question.
if not disable it after de-assignment, enable_ats_device will add the device multiple times to ats_devices list.


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