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Re: [PATCH v14 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty


From: Kirti Wankhede
Subject: Re: [PATCH v14 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty pages tracking.
Date: Fri, 20 Mar 2020 01:55:10 +0530
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0



On 3/19/2020 9:52 PM, Alex Williamson wrote:
On Thu, 19 Mar 2020 20:22:41 +0530
Kirti Wankhede <address@hidden> wrote:

On 3/19/2020 9:15 AM, Alex Williamson wrote:
On Thu, 19 Mar 2020 01:11:11 +0530
Kirti Wankhede <address@hidden> wrote:

<snip>

+
+static int verify_bitmap_size(uint64_t npages, uint64_t bitmap_size)
+{
+       uint64_t bsize;
+
+       if (!npages || !bitmap_size || bitmap_size > UINT_MAX)

As commented previously, how do we derive this UINT_MAX limitation?

Sorry, I missed that earlier

  > UINT_MAX seems arbitrary, is this specified in our API?  The size of a
  > vfio_dma is limited to what the user is able to pin, and therefore
  > their locked memory limit, but do we have an explicit limit elsewhere
  > that results in this limit here.  I think a 4GB bitmap would track
  > something like 2^47 bytes of memory, that's pretty excessive, but still
  > an arbitrary limit.

There has to be some upper limit check. In core KVM, in
virt/kvm/kvm_main.c there is max number of pages check:

if (new.npages > KVM_MEM_MAX_NR_PAGES)

Where
/*
   * Some of the bitops functions do not support too long bitmaps.
   * This number must be determined not to exceed such limits.
   */
#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)

Though I don't know which bitops functions do not support long bitmaps.

Something similar as above can be done or same as you also mentioned of
4GB bitmap limit? that is U32_MAX instead of UINT_MAX?

Let's see, we use bitmap_set():

void bitmap_set(unsigned long *map, unsigned int start, unsigned int nbits)

So we're limited to an unsigned int number of bits, but for an
unaligned, multi-bit operation this will call __bitmap_set():

void __bitmap_set(unsigned long *map, unsigned int start, int len)

So we're down to a signed int number of bits (seems like an API bug in
bitops there), so it makes sense that KVM is testing against MAX_INT
number of pages, ie. number of bits.  But that still suggests a bitmap
size of MAX_UINT is off by a factor of 16.  So we can have 2^31 bits
divided by 2^3 bits/byte yields a maximum bitmap size of 2^28 (ie.
256MB), which maps 2^31 * 2^12 = 2^43 (8TB) on a 4K system.

Let's fix the limit check and put a nice comment explaining it.  Thanks,


Agreed. Adding DIRTY_BITMAP_SIZE_MAX macro and comment as below.

/*
* 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  ((1UL << 31) - 1)
#define DIRTY_BITMAP_SIZE_MAX   \
                        DIRTY_BITMAP_BYTES(DIRTY_BITMAP_PAGES_MAX)


Thanks,
Kirti



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