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Re: Bug in pci-detach (Was: Re: [Xen-devel] Re: VT-d HowTo Update)

To: Simon Horman <horms@xxxxxxxxxxxx>
Subject: Re: Bug in pci-detach (Was: Re: [Xen-devel] Re: VT-d HowTo Update)
From: "Zhai, Edwin" <edwin.zhai@xxxxxxxxx>
Date: Wed, 22 Apr 2009 11:37:30 +0800
Cc: "Han, Weidong" <weidong.han@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Christian Tramnitz <chris.ace@xxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Cui, Dexuan" <dexuan.cui@xxxxxxxxx>
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Simon Horman wrote:
Hi,

I've poked a little further into this and the problem is partly
a missunderstanding on my part.

In the case where a function is specified in the hvm.conf in
the form BUS:DEV:FUNC (e.g. 00:1d.0), then there is a check inside
xm that prevents the function from subsequently being hot-unplugged.
The check is that the vslot number is set to auto-select. And presumably
the idea was that statically assigned devices couldn't be hot-unplugged,
which to be honest was my initial assumption of how things work.

All assigned device should not has a "auto-select" vslot after being assigned to guest, so I'm curious which lead this "auto-select". Static assigned device also support hotplug, because qemu will select then return the vslot to xend.



If, however, the function in the hvm.conf is of the form BUS:DEV:FUNC@VSLOT
(e.g. 00:1d.00@1e) which I added recently, then the check is bypassed,
because vslot is set to 1e not auto. This is how I was doing my tests. I
assumed this was the behaviour for all cases, and documented it as such.

This leaves us with 3 choices:

1. Leave the inconsistency, which seems like a bad idea.
2. Add a check to make sure both forms can't be hot-unplugged
   This is more work than;
3. Remove the check and allow statically assigned devies to be removed.
   This seems like a good option, and if there is consensus I can
   fix this up into a propper patch.

diff -r 6ba4e34d21d3 tools/python/xen/xend/XendDomainInfo.py
--- a/tools/python/xen/xend/XendDomainInfo.py   Tue Apr 21 16:31:26 2009 +0100
+++ b/tools/python/xen/xend/XendDomainInfo.py   Wed Apr 22 12:42:58 2009 +1000
@@ -1092,9 +1092,6 @@
         if devnum >= pci_len:
             raise VmError("Device @ vslot 0x%x doesn't exist." % (vslot))
- if vslot == AUTO_PHP_SLOT:
-            raise VmError("Device @ vslot 0x%x doesn't support hotplug." % 
(vslot))
-
         # Check the co-assignment.
         # To pci-detach a device D from domN, we should ensure: for each DD in 
the
         # list of D's co-assignment devices, DD is not assigned (to domN).


--
best rgds,
edwin


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