[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 04/37] xen: introduce an arch helper for default dma zone status
On 18.01.2022 10:20, Wei Chen wrote: >> From: Jan Beulich <jbeulich@xxxxxxxx> >> Sent: 2022年1月18日 16:16 >> >> On 18.01.2022 08:51, Wei Chen wrote: >>>> From: Jan Beulich <jbeulich@xxxxxxxx> >>>> Sent: 2022年1月18日 0:11 >>>> On 23.09.2021 14:02, 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 for DMA. >>>>> >>>>> There are some x86 implications in current implementation. Becuase >>>>> 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 >>>>> prefer to give out memory from non-local allocations rather than >>>>> exhausting the DMA memory ranges. Hence x86 use dma_bitsize to set >>>>> aside some largely arbitrary amount memory for DMA memory ranges. >>>>> The allocations from these memory ranges would happen only after >>>>> exhausting all other nodes' memory. >>>>> >>>>> But the implications are not shared across all architectures. For >>>>> example, Arm doesn't have these implications. So in this patch, we >>>>> introduce an arch_have_default_dmazone helper for arch to determine >>>>> that it need to set dma_bitsize for reserve DMA allocations or not. >>>> >>>> How would Arm guarantee availability of memory below a certain >>>> boundary for limited-capability devices? Or is there no need >>>> because there's an assumption that I/O for such devices would >>>> always pass through an IOMMU, lifting address size restrictions? >>>> (I guess in a !PV build on x86 we could also get rid of such a >>>> reservation.) >>> >>> On Arm, we still can have some devices with limited DMA capability. >>> And we also don't force all such devices to use IOMMU. This devices >>> will affect the dma_bitsize. Like RPi platform, it sets its dma_bitsize >>> to 30. But in multiple NUMA nodes system, Arm doesn't have a default >>> DMA zone. Multiple nodes is not a constraint on dma_bitsize. And some >>> previous discussions are placed here [1]. >> >> I'm afraid that doesn't give me more clues. For example, in the mail >> being replied to there I find "That means, only first 4GB memory can >> be used for DMA." Yet that's not an implication from setting >> dma_bitsize. DMA is fine to occur to any address. The special address >> range is being held back in case in particular Dom0 is in need of such >> a range to perform I/O to _some_ devices. > > I am sorry that my last reply hasn't given you more clues. On Arm, only > Dom0 can have DMA without IOMMU. So when we allocate memory for Dom0, > we're trying to allocate memory under 4GB or in the range of dma_bitsize > indicated. I think these operations meet your above Dom0 special address > range description. As we have already allocated memory for DMA, so I > think we don't need a DMA zone in page allocation. I am not sure is that > answers your earlier question? I view all of this as flawed, or as a workaround at best. Xen shouldn't make assumptions on what Dom0 may need. Instead Dom0 should make arrangements such that it can do I/O to/from all devices of interest. This may involve arranging for address restricted buffers. And for this to be possible, Xen would need to have available some suitable memory. I understand this is complicated by the fact that despite being HVM-like, due to the lack of an IOMMU in front of certain devices address restrictions on Dom0 address space alone (i.e. without any Xen involvement) won't help ... Jan
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