Re: [PATCH] Enable non page boundary BAR device assignment

[Avi Kivity <avi@redhat.com>]

On 12/09/2009 11:06 PM, Alexander Graf wrote:
>
> Am 09.12.2009 um 21:49 schrieb "Michael S. Tsirkin" <mst@redhat.com>:
>
>> On Wed, Dec 09, 2009 at 06:38:54PM +0100, Alexander Graf wrote:
>>> While trying to get device passthrough working with an emulex hba, kvm
>>> refused to pass it through because it has a BAR of 256 bytes:
>>>
>>>        Region 0: Memory at d2100000 (64-bit, non-prefetchable) 
>>> [size=4K]
>>>        Region 2: Memory at d2101000 (64-bit, non-prefetchable) 
>>> [size=256]
>>>        Region 4: I/O ports at b100 [size=256]
>>>
>>> Since the page boundary is an arbitrary optimization to allow 1:1 
>>> mapping of
>>> physical to virtual addresses, we can still take the old MMIO 
>>> callback route.
>>>
>>> So let's add a second code path that allows for size & 0xFFF != 0 
>>> sized regions
>>> by looping it through userspace.
>>>
>>> I verified that it works by passing through an e1000 with this 
>>> additional patch
>>> applied and the card acted the same way it did without this patch:
>>>
>>>             map_func = assigned_dev_iomem_map;
>>> -            if (cur_region->size & 0xFFF) {
>>> +            if (i != PCI_ROM_SLOT){
>>>                 fprintf(stderr, "PCI region %d at address 0x%llx "
>>>
>>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>
>> Good idea.
>>
>> One thing I am concerned with, is that some users might
>> see performance degradation and not see the error message.
>> Maybe we can have a flag to optionally fail unless
>> passthrough can be fast? I'm not really sure it's worth it
>> to add such a flag though - what do others think?
>
> It only kicks in on small BARs which are usually not _that_ 
> performance critical. We're mostly talking about ack and status bits.
>

Well, ack and status are pretty important if accessed every interrupt.

> Also not failing > failing IMHO :). Even if it's not as fast as native.

Yes.

-- 
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