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

To: "Zhai, Edwin" <edwin.zhai@xxxxxxxxx>
Subject: Re: Bug in pci-detach (Was: Re: [Xen-devel] Re: VT-d HowTo Update)
From: Simon Horman <horms@xxxxxxxxxxxx>
Date: Wed, 22 Apr 2009 14:13:07 +1000
Cc: Christian Tramnitz <chris.ace@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Han, Weidong" <weidong.han@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Cui, Dexuan" <dexuan.cui@xxxxxxxxx>
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On Wed, Apr 22, 2009 at 11:37:30AM +0800, Zhai, Edwin wrote:
> 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.

That is what I thought too, I'll investigate further.

>> 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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Simon Horman
  VA Linux Systems Japan K.K. Satellite Lab in Sydney, Australia
  H: www.vergenet.net/~horms/            W: www.valinux.co.jp/en

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