Re: [RFC PATCH kernel] vfio-pci: Fix sparse capability when no parts of MSIX BAR can be mapped

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 30/11/17 05:27, Alex Williamson wrote:
> On Thu, 23 Nov 2017 15:56:26 +1100
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> It is currently possible to have a sparse capability with 1 areas which
>> starts at 0 and 0 bytes long. One example is:
>>
>> Texas Instruments TUSB73x0 SuperSpeed USB 3.0 xHCI Host Controller
>> [...]
>> Region 0: Memory at 3fe280000000 (64-bit, non-prefetchable) [size=64K]
>> Region 2: Memory at 3fe280010000 (64-bit, non-prefetchable) [size=8K]
>> [...]
>> Capabilities: [c0] MSI-X: Enable+ Count=8 Masked-
>>         Vector table: BAR=2 offset=00000000
>>         PBA: BAR=2 offset=00001000
>>
>> With PAGE_SIZE=64K, MSIX BAR occupies the entire BAR2 and cannot be
>> mapped.
>>
>> This makes it explicit - if sparse->areas is empty, then advertise
>> nr_areas as 0.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>
>> QEMU gets it right as vfio_setup_region_sparse_mmaps() checks for size
>> after QEMU's 24acf72b9a291ce "vfio: Handle zero-length sparse mmap ranges"
>> but why not make it explicit in the first place?
>>
>>
>> ---
>>  drivers/vfio/pci/vfio_pci.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index f041b1a..a201c45 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
>> @@ -597,6 +597,10 @@ static int msix_sparse_mmap_cap(struct vfio_pci_device *vdev,
>>  		i++;
>>  	}
>>  
>> +	/* With all alignments, there are no gaps left to mmap */
>> +	if (i == 0)
>> +		sparse->nr_areas = 0;
>> +
> 
> Ok, but why does 0 become a special case?  Shouldn't we set
> sparse->nr_areas = i?  Thanks,

This is what is returned to QEMU now - 1 sparse region, starts at 0, 0
bytes long. I am missing the point in having such region...


> 
> Alex
> 
>>  	ret = vfio_info_add_capability(caps, VFIO_REGION_INFO_CAP_SPARSE_MMAP,
>>  				       sparse);
>>  	kfree(sparse);
> 


-- 
Alexey