Re: [Qemu-devel] [PATCH] qcow2: Fix the calculation of the maximum L2 cache size

[Alberto Garcia]

On Fri 16 Aug 2019 02:59:21 PM CEST, Kevin Wolf wrote:
> The requirement so that this bug doesn't affect the user seems to be
> that the image size is a multiple of 64k * 8k = 512 MB. Which means
> that users are probably often lucky enough in practice.

Or rather: cluster_size^2 / 8, which, if my numbers are right:

|--------------+-------------|
| Cluster size | Multiple of |
|--------------+-------------|
|         4 KB |        2 MB |
|         8 KB |        8 MB |
|        16 KB |       32 MB |
|        32 KB |      128 MB |
|        64 KB |      512 MB |
|       128 KB |        2 GB |
|       256 KB |        8 GB |
|       512 KB |       32 GB |
|      1024 KB |      128 GB |
|      2048 KB |      512 GB |
|--------------+-------------|

It get trickier with larger clusters, but if you have a larger cluster
size you probably have a very large image anyway, so yes I also think
that users are probably lucky enough in practice.

(also, the number of cache tables is always >= 2, so if the image size
is less than twice those numbers then it's also safe)

And yes, the odd value on the 512KB row on that we discussed last month
is due to this same bug:

https://lists.gnu.org/archive/html/qemu-block/2019-07/msg00496.html

Berto