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Re: [XEN RFC PATCH 05/40] xen/arm: Fix lowmem_bitsize when arch_get_dma_bitsize return 0


I guess this patch may be dropped after my comment on patch #4. I will comment just on the process.

On 11/08/2021 11:23, Wei Chen wrote:
From: Hongda Deng <Hongda.Deng@xxxxxxx>

In previous patch, we make arch_get_dma_bitsize return 0 when
dma_bitsize and platform->dma_bitsize are not set. But this
will affect lowmem_bitsize in allocate_memory_11 for domain0.
Because this function depends lowmem_bitsize to allocate memory
below 4GB.

In current code, when arch_get_dma_bitsize return 0, lowmem_bitsize
will be set to 0. In this case, we will get "No bank has been
allocated below 0-bit." message while allocating domain0 memory.
And the lowmem will be set to false.

This behavior is inconsistent with what allocate_memory_11 done
before, and doesn't meet this functions requirements. So we
check arch_get_dma_bitsize's return value before set lowmem_bitsize.
Avoid setting lowmem_bitsize to 0 by mistake.

In general, we want to avoid breaking bisection within a series. This means that this patch should be before patch #4.

Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
Signed-off-by: Hongda Deng <Hongda.Deng@xxxxxxx>
  xen/arch/arm/domain_build.c | 11 ++++++++++-
  1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index 6c86d52781..cf341f349f 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -265,9 +265,18 @@ static void __init allocate_memory_11(struct domain *d,
      int i;
bool lowmem = true;
-    unsigned int lowmem_bitsize = min(32U, arch_get_dma_bitsize());
+    unsigned int lowmem_bitsize = arch_get_dma_bitsize();
      unsigned int bits;
+ /*
+       When dma_bitsize and platform->dma_bitsize are not set,
+       arch_get_dma_bitsize will return 0. That means this system
+       doesn't need to reserve memory for DMA. But in order to
+       meet above requirements, we still need to try to allocate
+       memory below 4GB for Dom0.
+    */

The coding style for comments is:

 * A
 * B

+    lowmem_bitsize = lowmem_bitsize ? min(32U, lowmem_bitsize) : 32U;
       * TODO: Implement memory bank allocation when DOM0 is not direct
       * mapped


Julien Grall



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