Re: [PATCH v17 Kernel 6/7] vfio iommu: Adds flag to indicate dirty pages tracking capability support

[Kirti Wankhede <kwankhede@nvidia.com>]

On 4/1/2020 12:45 AM, Alex Williamson wrote:
> On Wed, 1 Apr 2020 00:38:49 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 3/31/2020 2:28 AM, Alex Williamson wrote:
>>> On Mon, 30 Mar 2020 22:20:43 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>    
>>>> Flag VFIO_IOMMU_INFO_DIRTY_PGS in VFIO_IOMMU_GET_INFO indicates that driver
>>>> support dirty pages tracking.
>>>>
>>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>>>> ---
>>>>    drivers/vfio/vfio_iommu_type1.c | 3 ++-
>>>>    include/uapi/linux/vfio.h       | 5 +++--
>>>>    2 files changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 266550bd7307..9fe12b425976 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -2390,7 +2390,8 @@ static long vfio_iommu_type1_ioctl(void *iommu_data,
>>>>    			info.cap_offset = 0; /* output, no-recopy necessary */
>>>>    		}
>>>>    
>>>> -		info.flags = VFIO_IOMMU_INFO_PGSIZES;
>>>> +		info.flags = VFIO_IOMMU_INFO_PGSIZES |
>>>> +			     VFIO_IOMMU_INFO_DIRTY_PGS;
>>>>    
>>>>    		info.iova_pgsizes = vfio_pgsize_bitmap(iommu);
>>>>    
>>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>>> index e3cbf8b78623..0fe7c9a6f211 100644
>>>> --- a/include/uapi/linux/vfio.h
>>>> +++ b/include/uapi/linux/vfio.h
>>>> @@ -985,8 +985,9 @@ struct vfio_device_feature {
>>>>    struct vfio_iommu_type1_info {
>>>>    	__u32	argsz;
>>>>    	__u32	flags;
>>>> -#define VFIO_IOMMU_INFO_PGSIZES (1 << 0)	/* supported page sizes info */
>>>> -#define VFIO_IOMMU_INFO_CAPS	(1 << 1)	/* Info supports caps */
>>>> +#define VFIO_IOMMU_INFO_PGSIZES   (1 << 0) /* supported page sizes info */
>>>> +#define VFIO_IOMMU_INFO_CAPS      (1 << 1) /* Info supports caps */
>>>> +#define VFIO_IOMMU_INFO_DIRTY_PGS (1 << 2) /* supports dirty page tracking */
>>>>    	__u64	iova_pgsizes;	/* Bitmap of supported page sizes */
>>>>    	__u32   cap_offset;	/* Offset within info struct of first cap */
>>>>    };
>>>
>>>
>>> As I just mentioned in my reply to Yan, I'm wondering if
>>> VFIO_CHECK_EXTENSION would be a better way to expose this.  The
>>> difference is relatively trivial, but currently the only flag
>>> set by VFIO_IOMMU_GET_INFO is to indicate the presence of a field in
>>> the returned structure.  I think this is largely true of other INFO
>>> ioctls within vfio as well and we're already using the
>>> VFIO_CHECK_EXTENSION ioctl to check supported IOMMU models, and IOMMU
>>> cache coherency.  We'd simply need to define a VFIO_DIRTY_PGS_IOMMU
>>> value (9) and return 1 for that case.  Then when we enable support for
>>> dirt pages that can span multiple mappings, we can add a v2 extensions,
>>> or "MULTI" variant of this extension, since it should be backwards
>>> compatible.
>>>
>>> The v2/multi version will again require that the user provide a zero'd
>>> bitmap, but I don't think that should be a problem as part of the
>>> definition of that version (we won't know if the user is using v1 or
>>> v2, but a v1 user should only retrieve bitmaps that exactly match
>>> existing mappings, where all bits will be written).  Thanks,
>>>
>>> Alex
>>>    
>>
>> I look at these two ioctls as : VFIO_CHECK_EXTENSION is used to get
>> IOMMU type, while VFIO_IOMMU_GET_INFO is used to get properties of a
>> particular IOMMU type, right?
> 
> Not exclusively, see for example VFIO_DMA_CC_IOMMU,
> 
>> Then I think VFIO_IOMMU_INFO_DIRTY_PGS should be part of
>> VFIO_IOMMU_GET_INFO and when we add code for v2/multi, a flag should be
>> added to VFIO_IOMMU_GET_INFO.
> 
> Which burns through flags, which is a far more limited resource than
> our 32bit extension address space, especially when we're already
> planning for one or more extensions to this support.  Thanks,
> 

To use flag from VFIO_IOMMU_GET_INFO was your original suggestion, only 
3 bits are used here as of now.

Thanks,
Kirti