[Xen-devel] [BUG?] limit too low in privcmd-buf.c

[Stefano Stabellini <sstabellini@kernel.org>]

Hi Juergen,

We are working on a technology to limit cache interference between
guests running on the same SoC. It works well, but as a consequence, all
memory allocations are 4K only: higher granularities (2M, 1G) do not
work at all.

One of the issues I am seeing after upgrading dom0 kernel is that the
limit in privcmd-buf.c, which is 64, is too low.  (Previously I was
using an old kernel without privcmd-buf.c.) The issue is that I cannot
start a guest of 128MB because mmap returns an error, specifically the
following check in privcmd_buf_mmap:privcmd_buf_mmap fails:

        if (!(vma->vm_flags & VM_SHARED) || count > limit ||
            file_priv->allocated + count > limit)
            return -EINVAL;

The check to fail is "file_priv->allocated + count > limit".

In my tests, limit needs to be roughly similar to the amount of MB of
guest RAM given to the guest. For example, we want a limit of 512 to be
able to start a guest with 512MB of RAM.

For the moment, I solved the problem simply by increasing the limit.
However, I don't know for sure what the right limit would be and whether
it is the right way to solve the problem.

Do you have any suggestions?

Cheers,

Stefano

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