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Re: [PATCH] device-tree: Improve hwdom memory allocation for DMA
- To: Michal Orzel <michal.orzel@xxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Mon, 20 Apr 2026 14:54:56 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- Delivery-date: Mon, 20 Apr 2026 14:56:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH] device-tree: Improve hwdom memory allocation for DMA
Hi Michal,
> On 17 Apr 2026, at 10:11, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>
> When LLC coloring is enabled, the hardware domain gets memory from
> host free regions rather than the fixed guest RAM banks. The previous
> code sorted these regions by descending size, which usually causes
> high-address memory to be allocated first.
>
> All allocated memory could reside above 4 GB leaving DMA non-functional
> for devices with limited addressing capabilities.
>
> Improve the handling as follows:
> - Sort free regions by ascending address instead of descending size,
> so low-memory banks are allocated first,
> - Skip banks smaller than 128 MB (or the total remaining allocation,
> whichever is less) until the first bank is placed, ensuring
> place_modules() has enough contiguous space,
> - Extract the hardware domain allocation path into its own function
> (allocate_hwdom_memory) for clarity.
>
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
> xen/common/device-tree/domain-build.c | 152 ++++++++++++++++----------
> 1 file changed, 96 insertions(+), 56 deletions(-)
It looks ok to me, the only thing is that if we have many smaller banks < 128
Mb before reaching
one that is at least 128 Mb, we won’t allocate them and loose them forever.
It feels only a corner case so for me it’s ok
Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx
<mailto:luca.fancellu@xxxxxxx>>
Cheers,
Luca
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