Re: [PATCH v3 4/9] xen: introduce an arch helper for default dma zone status

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.05.2022 03:46, Wei Chen wrote:
> In current code, when Xen is running in a multiple nodes
> NUMA system, it will set dma_bitsize in end_boot_allocator
> to reserve some low address memory as DMA zone.
> 
> There are some x86 implications in the implementation.
> Because on x86, memory starts from 0. On a multiple-nodes
> NUMA system, if a single node contains the majority or all
> of the DMA memory, x86 prefers to give out memory from
> non-local allocations rather than exhausting the DMA memory
> ranges. Hence x86 uses dma_bitsize to set aside some largely
> arbitrary amount of memory for DMA zone. The allocations
> from DMA zone would happen only after exhausting all other
> nodes' memory.
> 
> But the implications are not shared across all architectures.
> For example, Arm cannot guarantee the availability of memory
> below a certain boundary for DMA limited-capability devices
> either. But currently, Arm doesn't need a reserved DMA zone
> in Xen. Because there is no DMA device in Xen. And for guests,
> Xen Arm only allows Dom0 to have DMA operations without IOMMU.
> Xen will try to allocate memory under 4GB or memory range that
> is limited by dma_bitsize for Dom0 in boot time. For DomU, even
> Xen can passthrough devices to DomU without IOMMU, but Xen Arm
> doesn't guarantee their DMA operations. So, Xen Arm doesn't
> need a reserved DMA zone to provide DMA memory for guests.
> 
> In this patch, we introduce an arch_want_default_dmazone helper
> for different architectures to determine whether they need to
> set dma_bitsize for DMA zone reservation or not.
> 
> At the same time, when x86 Xen is built with CONFIG_PV=n could
> probably leverage this new helper to actually not trigger DMA
> zone reservation.
> 
> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> Tested-by: Jiamei Xie <jiamei.xie@xxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>