Hi, Zenghui,
On Tue, Dec 08, 2020 at 07:40:13PM +0800, Zenghui Yu wrote:
> The kernel KVM_CLEAR_DIRTY_LOG interface has align requirement on both the
> start and the size of the given range of pages. We have been careful to
> handle the unaligned cases when performing CLEAR on one slot. But it seems
> that we forget to take the unaligned *size* case into account when
> preparing bitmap for the interface, and we may end up clearing dirty status
> for pages outside of [start, start + size).
Thanks for the patch, though my understanding is that this is not a bug.
Please have a look at kvm_memslot_init_dirty_bitmap() where we'll allocate the
dirty bitmap to be aligned to 8 bytes (assuming that's the possible max of the
value sizeof(unsigned long)). That exactly covers 64 pages.
So here as long as start_delta==0 (so the value of "bmap_npages - size / psize"
won't really matter a lot, imho), then we'll definitely have KVMSlot.dirty_bmap
long enough to cover the range we'd like to clear.
Note that the size of KVMSlot.dirty_bmap can be bigger than the actually size
of the kvm memslot, however since kvm_memslot_init_dirty_bitmap() initialized
it to all zero so the extra bits will always be zero for the whole lifecycle of
the vm/bitmap.
Thanks,
>
> Signed-off-by: Zenghui Yu <yuzeng...@huawei.com>
> ---
> accel/kvm/kvm-all.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index bed2455ca5..05d323ba1f 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -747,7 +747,7 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int
> as_id, uint64_t start,
> assert(bmap_start % BITS_PER_LONG == 0);
> /* We should never do log_clear before log_sync */
> assert(mem->dirty_bmap);
> - if (start_delta) {
> + if (start_delta || bmap_npages - size / psize) {
> /* Slow path - we need to manipulate a temp bitmap */
> bmap_clear = bitmap_new(bmap_npages);
> bitmap_copy_with_src_offset(bmap_clear, mem->dirty_bmap,
> @@ -760,7 +760,10 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int
> as_id, uint64_t start,
> bitmap_clear(bmap_clear, 0, start_delta);
> d.dirty_bitmap = bmap_clear;
> } else {
> - /* Fast path - start address aligns well with BITS_PER_LONG */
> + /*
> + * Fast path - both start and size align well with BITS_PER_LONG
> + * (or the end of memory slot)
> + */
> d.dirty_bitmap = mem->dirty_bmap + BIT_WORD(bmap_start);
> }
>
> --
> 2.19.1
>
>
--
Peter Xu
