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

[Alex Williamson]

On Mon, 30 Mar 2020 19:51:31 -0400
Yan Zhao <address@hidden> wrote:

> 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?

If your concern is that the IOMMU supports sub-4K page sizes, see
vfio_pgsize_bitmap().  We actually only support PAGE_SIZE as our
minimum mapping unit, even if the IOMMU supports less, so PAGE_SIZE is
our lower bound.  Thanks,

Alex