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

[Yan Zhao <yan.y.zhao@intel.com>]

On Tue, Mar 31, 2020 at 09:12:59AM +0800, Alex Williamson wrote:
> On Mon, 30 Mar 2020 20:50:47 -0400
> Yan Zhao <yan.y.zhao@intel.com> wrote:
> 
> > On Tue, Mar 31, 2020 at 08:53:47AM +0800, Alex Williamson wrote:
> > > On Mon, 30 Mar 2020 19:51:31 -0400
> > > Yan Zhao <yan.y.zhao@intel.com> 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 <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.
> > > > >     
> > > > 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,  
> > 
> > if we always uses PAGE_SIZE, why not use PAGE_SIZE directly?
> > or returning dirty bitmap unit (e.g. 1 << __ffs(vfio_pgsize_bitmap(iommu)))
> > to QEMU in VFIO_IOMMU_DIRTY_PAGES_FLAG_START, so that qemu can do possible
> > conversion if it's not the same unit that QEMU uses.
> 
> The vfio interface is essentially just an extension of the IOMMU API
> via domain->pgsize_bitmap.  intel-iommu mostly made the bitmask
> meaningless by reporting essentially PAGE_MASK, and we just expose the
> common version of that across potentially all the IOMMUs used by the
> domain, modulo minimum of PAGE_SIZE.  Thanks,

ok. got it. do you think it's good to return this iommu page size
when turning on dirty page tracking? so when GET_BITMAP ioctl
comes, we don't need to quit if range.bitmap.pgsize != iommu_pgsize.
instead, the GET_BITMAP can success with iommu page size and qemu does
the bitmap conversion afterwards.

Thanks
Yan