Re: [PATCH v12 Kernel 4/7] vfio iommu: Implementation of ioctl to for dirty pages tracking.

Alex Williamson
2020-02-13T23:20:24Z

On Fri, 14 Feb 2020 01:41:35 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> <snip>
>
> >>>>
> >>>> +static int vfio_iova_dirty_bitmap(struct vfio_iommu *iommu, dma_addr_t iova,
> >>>> + size_t size, uint64_t pgsize,
> >>>> + unsigned char __user *bitmap)
> >>>> +{
> >>>> + struct vfio_dma *dma;
> >>>> + dma_addr_t i = iova, iova_limit;
> >>>> + unsigned int bsize, nbits = 0, l = 0;
> >>>> + unsigned long pgshift = __ffs(pgsize);
> >>>> +
> >>>> + while ((dma = vfio_find_dma(iommu, i, pgsize))) {
> >>>> + int ret, j;
> >>>> + unsigned int npages = 0, shift = 0;
> >>>> + unsigned char temp = 0;
> >>>> +
> >>>> + /* mark all pages dirty if all pages are pinned and mapped. */
> >>>> + if (dma->iommu_mapped) {
> >>>> + iova_limit = min(dma->iova + dma->size, iova + size);
> >>>> + npages = iova_limit/pgsize;
> >>>> + bitmap_set(dma->bitmap, 0, npages);
> >>>
> >>> npages is derived from iova_limit, which is the number of bits to set
> >>> dirty relative to the first requested iova, not iova zero, ie. the set
> >>> of dirty bits is offset from those requested unless iova == dma->iova.
> >>>
> >>
> >> Right, fixing.
> >>
> >>> Also I hope dma->bitmap was actually allocated. Not only does the
> >>> START error path potentially leave dirty tracking enabled without all
> >>> the bitmap allocated, when does the bitmap get allocated for a new
> >>> vfio_dma when dirty tracking is enabled? Seems it only occurs if a
> >>> vpfn gets marked dirty.
> >>>
> >>
> >> Right.
> >>
> >> Fixing error paths.
> >>
> >>
> >>>> + } else if (dma->bitmap) {
> >>>> + struct rb_node *n = rb_first(&dma->pfn_list);
> >>>> + bool found = false;
> >>>> +
> >>>> + for (; n; n = rb_next(n)) {
> >>>> + struct vfio_pfn *vpfn = rb_entry(n,
> >>>> + struct vfio_pfn, node);
> >>>> + if (vpfn->iova >= i) {
> >>>> + found = true;
> >>>> + break;
> >>>> + }
> >>>> + }
> >>>> +
> >>>> + if (!found) {
> >>>> + i += dma->size;
> >>>> + continue;
> >>>> + }
> >>>> +
> >>>> + for (; n; n = rb_next(n)) {
> >>>> + unsigned int s;
> >>>> + struct vfio_pfn *vpfn = rb_entry(n,
> >>>> + struct vfio_pfn, node);
> >>>> +
> >>>> + if (vpfn->iova >= iova + size)
> >>>> + break;
> >>>> +
> >>>> + s = (vpfn->iova - dma->iova) >> pgshift;
> >>>> + bitmap_set(dma->bitmap, s, 1);
> >>>> +
> >>>> + iova_limit = vpfn->iova + pgsize;
> >>>> + }
> >>>> + npages = iova_limit/pgsize;
> >>>
> >>> Isn't iova_limit potentially uninitialized here? For example, if our
> >>> vfio_dma covers {0,8192} and we ask for the bitmap of {0,4096} and
> >>> there's a vpfn at {4096,8192}. I think that means vpfn->iova >= i
> >>> (4096 >= 0), so we break with found = true, then we test 4096 >= 0 +
> >>> 4096 and break, and npages = ????/pgsize.
> >>>
> >>
> >> Right, Fixing it.
> >>
> >>>> + }
> >>>> +
> >>>> + bsize = dirty_bitmap_bytes(npages);
> >>>> + shift = nbits % BITS_PER_BYTE;
> >>>> +
> >>>> + if (npages && shift) {
> >>>> + l--;
> >>>> + if (!access_ok((void __user *)bitmap + l,
> >>>> + sizeof(unsigned char)))
> >>>> + return -EINVAL;
> >>>> +
> >>>> + ret = __get_user(temp, bitmap + l);
> >>>
> >>> I don't understand why we care to get the user's bitmap, are we trying
> >>> to leave whatever garbage they might have set in it and only also set
> >>> the dirty bits? That seems unnecessary.
> >>>
> >>
> >> Suppose dma mapped ranges are {start, size}:
> >> {0, 0xa000}, {0xa000, 0x10000}
> >>
> >> Bitmap asked from 0 - 0x10000. Say suppose all pages are dirty.
> >> Then in first iteration for dma {0,0xa000} there are 10 pages, so 10
> >> bits are set, put_user() happens for 2 bytes, (00000011 11111111b).
> >> In second iteration for dma {0xa000, 0x10000} there are 6 pages and
> >> these bits should be appended to previous byte. So get_user() that byte,
> >> then shift-OR rest of the bitmap, result should be: (11111111 11111111b)
> >>
> >> Without get_user() and shift-OR, resulting bitmap would be
> >> 111111 00000011 11111111b which would be wrong.
> >
> > Seems like if we use a put_user() approach then we should look for
> > adjacent vfio_dmas within the same byte/word/dword before we push it to
> > the user to avoid this sort of inefficiency.
> >
>
> Won't that add more complication to logic?

I'm tempted to think it might be less complicated.

> >>> Also why do we need these access_ok() checks when we already checked
> >>> the range at the start of the ioctl?
> >>
> >> Since pointer is updated runtime here, better to check that pointer
> >> before using that pointer.
> >
> > Sorry, I still don't understand this, we check access_ok() with a
> > pointer and a length, therefore as long as we're incrementing the
> > pointer within that length, why do we need to retest?
> >
>
> Ideally caller for put_user() and get_user() must check the pointer with
> access_ok() which is used as argument to these functions before calling
> this function. That makes sure that pointer is correct after pointer
> arithematic. May be lets remove previous check of pointer and length,
> but keep these checks.

So we don't trust that we can increment a pointer within a range that
we've already tested with access_ok() and expect it to still be ok? I
think the point of having access_ok() and __put_user() is that we can
batch many __put_user() calls under a single access_ok() check. I
don't see any justification here why if we already tested
access_ok(ptr, 2) that we still need to test access_ok(ptr + 0, 1) and
access_ok(ptr + 1, 1), and removing the initial test is clearly the
wrong optimization if we agree there is redundancy here.

> >>>> + if (ret)
> >>>> + return ret;
> >>>> + }
> >>>> +
> >>>> + for (j = 0; j < bsize; j++, l++) {
> >>>> + temp = temp |
> >>>> + (*((unsigned char *)dma->bitmap + j) << shift);
> >>>
> >>> |=
> >>>
> >>>> + if (!access_ok((void __user *)bitmap + l,
> >>>> + sizeof(unsigned char)))
> >>>> + return -EINVAL;
> >>>> +
> >>>> + ret = __put_user(temp, bitmap + l);
> >>>> + if (ret)
> >>>> + return ret;
> >>>> + if (shift) {
> >>>> + temp = *((unsigned char *)dma->bitmap + j) >>
> >>>> + (BITS_PER_BYTE - shift);
> >>>> + }
> >>>
> >>> When shift == 0, temp just seems to accumulate bits that never get
> >>> cleared.
> >>>
> >>
> >> Hope example above explains the shift logic.
> >
> > But that example is when shift is non-zero. When shift is zero, each
> > iteration of the loop just ORs in new bits to temp without ever
> > clearing the bits for the previous iteration.
> >
> >
>
> Oh right, fixing it.
>
> >>>> + }
> >>>> +
> >>>> + nbits += npages;
> >>>> +
> >>>> + i = min(dma->iova + dma->size, iova + size);
> >>>> + if (i >= iova + size)
> >>>> + break;
> >>>
> >>> So whether we error or succeed, we leave cruft in dma->bitmap for the
> >>> next pass. It doesn't seem to make any sense why we pre-allocated the
> >>> bitmap, we might as well just allocate it on demand here. Actually, if
> >>> we're not going to do a copy_to_user() for some range of the bitmap,
> >>> I'm not sure what it's purpose is at all. I think the big advantages
> >>> of the bitmap are that we can't amortize the cost across every pinned
> >>> page or DMA mapping, we don't need the overhead of tracking unmapped
> >>> vpfns, and we can use copy_to_user() to push the bitmap out. We're not
> >>> getting any of those advantages here.
> >>>
> >>
> >> That would still not work if dma range size is not multiples of 8 pages.
> >> See example above.
> >
> > I don't understand this comment, what about the example above justifies
> > the bitmap?
>
> copy_to_user() could be used if dma range size is not multiple of 8 pages.

s/is not/is/ ?

And we expect that to be a far more common case, right? I don't think
there are too many ranges for a guest that are only mapped in sub-32KB
chucks.

> > As I understand the above algorithm, we find a vfio_dma
> > overlapping the request and populate the bitmap for that range. Then
> > we go back and put_user() for each byte that we touched. We could
> > instead simply work on a one byte buffer as we enumerate the requested
> > range and do a put_user() ever time we reach the end of it and have bits
> > set. That would greatly simplify the above example. But I would expect
> > that we're a) more likely to get asked for ranges covering a single
> > vfio_dma
>
> QEMU ask for single vfio_dma during each iteration.
>
> If we restrict this ABI to cover single vfio_dma only, then it
> simplifies the logic here. That was my original suggestion. Should we
> think about that again?

But we currently allow unmaps that overlap multiple vfio_dmas as long
as no vfio_dma is bisected, so I think that implies that an unmap while
asking for the dirty bitmap has even further restricted semantics. I'm
also reluctant to design an ABI around what happens to be the current
QEMU implementation.

If we take your example above, ranges {0x0000,0xa000} and
{0xa000,0x10000} ({start,end}), I think you're working with the
following two bitmaps in this implementation:

00000011 11111111b
00111111b

And we need to combine those into:

11111111 11111111b

Right?

But it seems like that would be easier if the second bitmap was instead:

11111100b

Then we wouldn't need to worry about the entire bitmap being shifted by
the bit offset within the byte, which limits our fixes to the boundary
byte and allows us to use copy_to_user() directly for the bulk of the
copy. So how do we get there?

I think we start with allocating the vfio_dma bitmap to account for
this initial offset, so we calculate bitmap_base_iova as:
(iova & ~((PAGE_SIZE << 3) - 1))
We then use bitmap_base_iova in calculating which bits to set.

The user needs to follow the same rules, and maybe this adds some value
to the user providing the bitmap size rather than the kernel
calculating it. For example, if the user wanted the dirty bitmap for
the range {0xa000,0x10000} above, they'd provide at least a 1 byte
bitmap, but we'd return bit #2 set to indicate 0xa000 is dirty.

Effectively the user can ask for any iova range, but the buffer will be
filled relative to the zeroth bit of the bitmap following the above
bitmap_base_iova formula (and replacing PAGE_SIZE with the user
requested pgsize). I'm tempted to make this explicit in the user
interface (ie. only allow bitmaps starting on aligned pages), but a
user is able to map and unmap single pages and we need to support
returning a dirty bitmap with an unmap, so I don't think we can do that.

So now are we biting off more than we can chew trying to transpose the
bitmap between page sizes? If asked for the previous range with an 8K
pgsize, we'd somehow need to translate 11111100b into 00001110b.
What's worse, the user could ask for just the 8K page at 0xa000 and we'd
need to return back 00000010b while leaving our internal bitmap a
11110000b after we mark the bits clean. Seems like this is really
only tenable if we do multiples of PAGE_SIZE pages within a byte, so
for 4K we'd have 32K, 64K, 128K, 256K, etc. I'm somewhat losing sight
on what this accomplishes though and whether we need this in the first
implementation. Should we simplify by dropping this aspect of it,
supporting only the minimum iommu page size, and focus on actually
using the bitmaps effectively?

> > and b) we're going to spend far more time operating in the
> > middle of the range and limiting ourselves to one-byte operations there
> > seems absurd. If we want to specify that the user provides 4-byte
> > aligned buffers and naturally aligned iova ranges to make our lives
> > easier in the kernel, now would be the time to do that.
> >
> >>>> + }
> >>>> + return 0;
> >>>> +}
> >>>> +
> >>>> +static long verify_bitmap_size(unsigned long npages, unsigned long bitmap_size)
> >>>> +{
> >>>> + long bsize;
> >>>> +
> >>>> + if (!bitmap_size || bitmap_size > SIZE_MAX)
> >>>> + return -EINVAL;
> >>>> +
> >>>> + bsize = dirty_bitmap_bytes(npages);
> >>>> +
> >>>> + if (bitmap_size < bsize)
> >>>> + return -EINVAL;
> >>>> +
> >>>> + return bsize;
> >>>> +}
> >>>
> >>> Seems like this could simply return int, -errno or zero for success.
> >>> The returned bsize is not used for anything else.
> >>>
> >>
> >> ok.
> >>
> >>>> +
> >>>> static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >>>> struct vfio_iommu_type1_dma_unmap *unmap)
> >>>> {
> >>>> @@ -2277,6 +2478,80 @@ static long vfio_iommu_type1_ioctl(void *iommu_data,
> >>>>
> >>>> return copy_to_user((void __user *)arg, &unmap, minsz) ?
> >>>> -EFAULT : 0;
> >>>> + } else if (cmd == VFIO_IOMMU_DIRTY_PAGES) {
> >>>> + struct vfio_iommu_type1_dirty_bitmap range;
> >>>> + uint32_t mask = VFIO_IOMMU_DIRTY_PAGES_FLAG_START |
> >>>> + VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP |
> >>>> + VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP;
> >>>> + int ret;
> >>>> +
> >>>> + if (!iommu->v2)
> >>>> + return -EACCES;
> >>>> +
> >>>> + minsz = offsetofend(struct vfio_iommu_type1_dirty_bitmap,
> >>>> + bitmap);
> >>>
> >>> We require the user to provide iova, size, pgsize, bitmap_size, and
> >>> bitmap fields to START/STOP? Why?
> >>>
> >>
> >> No. But those are part of structure.
> >
> > But we do require it, minsz here includes all those fields, which would
> > probably make a user scratch their head wondering why they need to pass
> > irrelevant data for START/STOP. It almost implies that we support
> > starting and stopping dirty logging for specific ranges of the IOVA
> > space. We could define the structure, for example:
> >
> > struct vfio_iommu_type1_dirty_bitmap {
> > __u32 argsz;
> > __u32 flags;
> > __u8 data[];
> > };
> >
> > struct vfio_iommu_type1_dirty_bitmap_get {
> > __u64 iova;
> > __u64 size;
> > __u64 pgsize;
> > __u64 bitmap_size;
> > void __user *bitmap;
> > };
> >
> > Where data[] is defined as the latter structure when FLAG_GET_BITMAP is
> > specified.
>
> Ok. Changing as above.
>
> > BTW, don't we need to specify the trailing void* as __u64?
> > We could theoretically be talking to an ILP32 user process. Thanks,
> >
>
> Even on ILP32, using void* pointer will reserve the size required to
> save a pointer address. I don't think using void* should be problem.

I think you're still assuming sizeof(void *) is the same in kernel vs
userspace whereas I'm thinking about an ILP32 user running on an LP64
kernel. Thanks,

Alex