Re: [PATCH v16 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty pages tracking.

[Kirti Wankhede <kwankhede@nvidia.com>]

On 3/30/2020 8:54 AM, Yan Zhao wrote:
> On Fri, Mar 27, 2020 at 01:28:13PM +0800, Kirti Wankhede wrote:
>> Hit send button little early.
>>
>>   >
>>   > I checked v12, it's not like what I said.
>>   > In v12, bitmaps are generated per vfio_dma, and combination of the
>>   > bitmaps are required in order to generate a big bitmap suiting for dirty
>>   > query. It can cause problem when offset not aligning.
>>   > But what I propose here is to generate an rb tree orthogonal to the tree
>>   > of vfio_dma.
>>   >
>>   > as to CPU cycles saving, I don't think iterating/translating page by page
>>   > would achieve that purpose.
>>   >
>>
>> Instead of creating one extra rb tree for dirty pages tracking in v10
>> tried to use dma->pfn_list itself, we tried changes in v10, v11 and v12,
>> latest version is evolved version with best possible approach after
>> discussion. Probably, go through v11 as well.
>> https://patchwork.kernel.org/patch/11298335/
>>
> I'm not sure why all those previous implementations are bound to
> vfio_dma. for vIOMMU on, in most cases, a vfio_dma is only for a page,
> so generating a one-byte bitmap for a single page in each vfio_dma ?
> is it possible to creating one extra rb tree to keep dirty ranges, and
> one fixed length kernel bitmap whose content is generated on query,
> serving as a bouncing buffer for copy_to_user
> 

One fixed length? what should be fixed value? then isn't it better to 
fix the size to dma->size?

This is also to prevent DoS attack, user space application can query a 
very large range.

>>
>> On 3/27/2020 6:00 AM, Yan Zhao wrote:
>>> On Fri, Mar 27, 2020 at 05:39:01AM +0800, Kirti Wankhede wrote:
>>>>
>>>>
>>>> On 3/25/2020 7:41 AM, Yan Zhao wrote:
>>>>> On Wed, Mar 25, 2020 at 05:18:52AM +0800, Kirti Wankhede wrote:
>>>>>> VFIO_IOMMU_DIRTY_PAGES ioctl performs three operations:
>>>>>> - Start dirty pages tracking while migration is active
>>>>>> - Stop dirty pages tracking.
>>>>>> - Get dirty pages bitmap. Its user space application's responsibility to
>>>>>>      copy content of dirty pages from source to destination during migration.
>>>>>>
>>>>>> To prevent DoS attack, memory for bitmap is allocated per vfio_dma
>>>>>> structure. Bitmap size is calculated considering smallest supported page
>>>>>> size. Bitmap is allocated for all vfio_dmas when dirty logging is enabled
>>>>>>
>>>>>> Bitmap is populated for already pinned pages when bitmap is allocated for
>>>>>> a vfio_dma with the smallest supported page size. Update bitmap from
>>>>>> pinning functions when tracking is enabled. When user application queries
>>>>>> bitmap, check if requested page size is same as page size used to
>>>>>> populated bitmap. If it is equal, copy bitmap, but if not equal, return
>>>>>> error.
>>>>>>
>>>>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>>>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>>>>>> ---
>>>>>>     drivers/vfio/vfio_iommu_type1.c | 266 +++++++++++++++++++++++++++++++++++++++-
>>>>>>     1 file changed, 260 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>>> index 70aeab921d0f..874a1a7ae925 100644
>>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>>> @@ -71,6 +71,7 @@ struct vfio_iommu {
>>>>>>     	unsigned int		dma_avail;
>>>>>>     	bool			v2;
>>>>>>     	bool			nesting;
>>>>>> +	bool			dirty_page_tracking;
>>>>>>     };
>>>>>>     
>>>>>>     struct vfio_domain {
>>>>>> @@ -91,6 +92,7 @@ struct vfio_dma {
>>>>>>     	bool			lock_cap;	/* capable(CAP_IPC_LOCK) */
>>>>>>     	struct task_struct	*task;
>>>>>>     	struct rb_root		pfn_list;	/* Ex-user pinned pfn list */
>>>>>> +	unsigned long		*bitmap;
>>>>>>     };
>>>>>>     
>>>>>>     struct vfio_group {
>>>>>> @@ -125,7 +127,21 @@ struct vfio_regions {
>>>>>>     #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)	\
>>>>>>     					(!list_empty(&iommu->domain_list))
>>>>>>     
>>>>>> +#define DIRTY_BITMAP_BYTES(n)	(ALIGN(n, BITS_PER_TYPE(u64)) / BITS_PER_BYTE)
>>>>>> +
>>>>>> +/*
>>>>>> + * Input argument of number of bits to bitmap_set() is unsigned integer, which
>>>>>> + * further casts to signed integer for unaligned multi-bit operation,
>>>>>> + * __bitmap_set().
>>>>>> + * Then maximum bitmap size supported is 2^31 bits divided by 2^3 bits/byte,
>>>>>> + * that is 2^28 (256 MB) which maps to 2^31 * 2^12 = 2^43 (8TB) on 4K page
>>>>>> + * system.
>>>>>> + */
>>>>>> +#define DIRTY_BITMAP_PAGES_MAX	(uint64_t)(INT_MAX - 1)
>>>>>> +#define DIRTY_BITMAP_SIZE_MAX	 DIRTY_BITMAP_BYTES(DIRTY_BITMAP_PAGES_MAX)
>>>>>> +
>>>>>>     static int put_pfn(unsigned long pfn, int prot);
>>>>>> +static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu);
>>>>>>     
>>>>>>     /*
>>>>>>      * This code handles mapping and unmapping of user data buffers
>>>>>> @@ -175,6 +191,77 @@ static void vfio_unlink_dma(struct vfio_iommu *iommu, struct vfio_dma *old)
>>>>>>     	rb_erase(&old->node, &iommu->dma_list);
>>>>>>     }
>>>>>>     
>>>>>> +
>>>>>> +static int vfio_dma_bitmap_alloc(struct vfio_dma *dma, uint64_t pgsize)
>>>>>> +{
>>>>>> +	uint64_t npages = dma->size / pgsize;
>>>>>> +
> If pgsize > dma->size, npages = 0.
> wouldn't it cause problem?
> 

This patch-set supports bitmap for smallest supported page size, i.e. 
PAGE_SIZE. vfio_dma_do_map() validates dma->size accordingly. So this 
case will not happen.

Thanks,
Kirti