Re: [Xen-devel] Dom0 hypercall for adding and removing PCI devices

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Can the pagetable switch not be made atomic (i.e., so that the RMRR regions
appear continuously available throughout)? I'd have thought that would just
naturally happen.

If creating/destroying RMRR mappings is part of the switch operation, you'd
have to destroy RMRR mappings in the dom0 table after the switch, and create
RMRR mappings in the fallback table before the switch. Or just have RMRR
mappings always mapped into all tables.

 -- Keir

On 24/7/08 09:32, "Han, Weidong" <weidong.han@xxxxxxxxx> wrote:

> We found USB (has RMRR) is firstly assigned to dom0, but pci_bus_probe()
> failed, then it was removed from dom0. The removing needs switch to RMRR
> VT-d page table. At the same time, BIOS was using its RMRR.
> 
> Randy (Weidong)
> 
> Keir Fraser wrote:
>> If a device is assigned to a domain (in this case dom0) then that
>> domain's VT-d pagetables will contain the RMRR mappings for that
>> device. Hence BIOS can perform DMA in those RMRR-indicated regions
>> without swapping to and fro between domain tables and fallback RMRR
>> tables. The new fallback RMRR table would be just that -- a fallback
>> table used for any device not currently assigned to any domain (and
>> hence those devices should only have DMAs initiated by the BIOS, if
>> at all). 
>> 
>>  -- Keir
>> 
>> On 24/7/08 09:20, "Han, Weidong" <weidong.han@xxxxxxxxx> wrote:
>> 
>>> I have another concern, when BIOS is initiating DMA operation using
>>> RMRR, can we use RMRR VT-d page table to replace dom0 VT-d page
>>> table? Does it result in some DMA failures?
>>> 
>>> Randy (weidong)
>>> 
>>> Han, Weidong wrote:
>>>> Espen Skoglund wrote:
>>>>> [Keir Fraser]
>>>>>> On 23/7/08 10:26, "Han, Weidong" <weidong.han@xxxxxxxxx> wrote:
>>>>>>>> So this would be one extra VT-d pagetable, for the whole system,
>>>>>>>> which would be the fallback location for RMRR mappings for
>>>>>>>> devices which are currently not assigned to any domain? Thus
>>>>>>>> allowing firmware to successfully initiate DMA operations on
>>>>>>>> those devices? Sounds sensible.
>>>>> 
>>>>> Well, the VT-d page table for RMRR devices need not contain the
>>>>> whole system memory---only those regions specified in the DMAR
>>>>> tables. 
>>>>> 
>>>>>>> Is it possible that idle_domain owns the RMRR VT-d page table?
>>>>> 
>>>>>> If that's a convenient place to stash it then why not? Either way,
>>>>>> seems you're going to have it special-cased in the code as
>>>>>> fallback owner for unassigned devices. It's possible that having
>>>>>> it stashed in the idle domain will simply make the code more
>>>>>> confusing. I'm not sure though.
>>>>> 
>>>>> Right.  I don't see any particular good reason to associate it with
>>>>> the idle domain.  As noted above, the page table need not even
>>>>> cover the whole memory, and it will never change after being set
>>>>> up at boot time.  If special case code is needed anyway, then one
>>>>> might as well install a custom VT-d page table.
>>>> 
>>>> What does "custom VT-d page table" mean?
>>>> 
>>>> Due to domain id is not the same with DID field in context, we can
>>>> reverse a DID for RMRR VT-d page table, it can avoid to associate
>>>> with idle domain.
>>>> 
>>>> Currently we reassign the device from dom0 to target domain when
>>>> assign a device, and return the device to dom0 when target domain
>>>> tears down. It's not correct due to some devices may be not assigned
>>>> to any domain. Current device_assigned() also needs to be changed.
>>>> Maybe it needs more changes on VT-d.
>>>> 
>>>> I have some concerns, maybe I missed something. Why did you use dom0
>>>> hypercall approach to replace original method? What's the main
>>>> benefit? I also wonder it's suitable to wrap pci_bus_probe()
>>>> function. 
>>>> 
>>>> Randy (Weidong)
>>>> 
>>>>> 
>>>>> If supported by hardware, the extra page tables can even be skipped
>>>>> altogether and the device marked as having passthrough access.
>>>>> That would give the RMRR device complete access to system memory,
>>>>> though. 
>>>>> 
>>>>> eSk
> 



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel