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

[Kirti Wankhede <kwankhede@nvidia.com>]

On 3/19/2020 9:52 PM, Alex Williamson wrote:
> On Thu, 19 Mar 2020 20:22:41 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 3/19/2020 9:15 AM, Alex Williamson wrote:
>>> On Thu, 19 Mar 2020 01:11:11 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> 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