On 19.01.2022 03:49, Wei Chen wrote: > Hi Jan, > >> -----Original Message----- >> From: Jan Beulich <[email protected]> >> Sent: 2022年1月18日 22:16 >> To: Wei Chen <[email protected]> >> Cc: Bertrand Marquis <[email protected]>; xen- >> [email protected]; [email protected]; [email protected] >> Subject: 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 <[email protected]> >>>> Sent: 2022年1月18日 16:16 >>>> >>>> On 18.01.2022 08:51, Wei Chen wrote: >>>>>> From: Jan Beulich <[email protected]> >>>>>> 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 ... >> > > I agree with you that the current implementation is probably the best > kind of workaround. Do you have some suggestions for this patch to > address above comments? Or should I just need to modify the commit log > to contain some of our above discussions?
Extending the description is my primary request, or else we may end up having the same discussion every time you submit a new version. As to improving the situation such that preferably per-arch customization wouldn't be necessary - that's perhaps better to be thought about by Arm folks. Otoh, as said, an x86 build with CONFIG_PV=n could probably leverage the new hook to actually not trigger reservation. Jan
