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Re: [Xen-devel] [BUG?] limit too low in privcmd-buf.c
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>, <jgross@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 22 Jul 2019 22:37:14 +0100
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- Delivery-date: Mon, 22 Jul 2019 21:37:30 +0000
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On 22/07/2019 22:21, Stefano Stabellini wrote:
> 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?
That limit was removed shortly afterwards (it regressed migrating of
medium sized VMs), and really ought to have hit stable by now...
~Andrew
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