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

[Yan Zhao]

On Mon, Mar 30, 2020 at 09:49:21PM +0800, Kirti Wankhede wrote:
> 
> 
> 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 <address@hidden>
> >>>>>> Reviewed-by: Neo Jia <address@hidden>
> >>>>>> ---
> >>>>>>     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.
> 
as far as I know, qemu/kvm uses 4k as the unit for dirty page tracking.
so why smallest iommu page size is used here?
wouldn't it cause problem?

Thanks
Yan