Hi Julien, > -----Original Message----- > From: Julien Grall <jul...@xen.org> > Sent: 2021年8月20日 19:18 > To: Wei Chen <wei.c...@arm.com>; xen-devel@lists.xenproject.org; > sstabell...@kernel.org > Cc: Bertrand Marquis <bertrand.marq...@arm.com> > Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit width > when platform is not set > > On 20/08/2021 10:37, Wei Chen wrote: > > Hi Julien, > > Hi Wei, > > >> -----Original Message----- > >> From: Julien Grall <jul...@xen.org> > >> Sent: 2021年8月20日 16:20 > >> To: Wei Chen <wei.c...@arm.com>; xen-devel@lists.xenproject.org; > >> sstabell...@kernel.org; jbeul...@suse.com > >> Cc: Bertrand Marquis <bertrand.marq...@arm.com> > >> Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit > width > >> when platform is not set > >> > >> > >> > >> On 20/08/2021 03:04, Wei Chen wrote: > >>> Hi Julien, > >> > >> Hi Wei, > >> > >>>> -----Original Message----- > >>>> From: Julien Grall <jul...@xen.org> > >>>> Sent: 2021年8月19日 21:28 > >>>> To: Wei Chen <wei.c...@arm.com>; xen-devel@lists.xenproject.org; > >>>> sstabell...@kernel.org; jbeul...@suse.com > >>>> Cc: Bertrand Marquis <bertrand.marq...@arm.com> > >>>> Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit > >> width > >>>> when platform is not set > >>>> > >>>> Hi, > >>>> > >>>> On 11/08/2021 11:23, Wei Chen wrote: > >>>>> From: Hongda Deng <hongda.d...@arm.com> > >>>>> > >>>>> In current code, arch_get_dma_bitsize will return 32 when platorm > >>>>> or platform->dma_bitsize is not set. It's not resonable, for Arm, > >>>> > >>>> s/resonable/reasonable/ > >>>> > >>> > >>> Ok > >>> > >>>>> we don't require to reserve DMA memory. So we set dma_bitsize always > >>>>> be 0. In NO-NUMA system, arch_get_dma_bitsize will not be invoked, > >>>>> so dma_bitsize will not be overrided by this function. > >>>> > >>>> arch_get_dma_bitsize() is also used to allocate dom0 memory. We need > to > >>>> be able to allocate some DMA-able memory that can be used by every > >> devices. > >>>> > >>>>> But in NUMA > >>>>> system, once the online nodes are greater than 1, this function will > >>>>> be invoked. The dma_bitsize will be limited to 32. That means, only > >>>>> first 4GB memory can be used for DMA. But that's against our > hardware > >>>>> design. We don't have that kind of restriction on hardware. > >>>> > >>>> What do you mean by "hardware design"? Are you referring to the > server > >>>> you boot Xen on? > >>>> > >>> > >>> Yes. I will change it to some neutral words. something like: > >>> "But that could not reflect some hardware's real DMA ability. They may > >> not > >>> have kind of restriction on hardware." ? > >> > >> The thing is DMA ability is not about the platform itself. It is more > >> about the devices (this could just be a PCI card you just plugged). > What > >> you seem to suggest is no-one will ever plug such card on your platform. > >> Is that correct? > >> > > > > OK, I understand now. Let's keep 32-bit as default value, but even in > this > > case, how about DMA-16 devices? Although these devices are very rare, > they > > still exist : ) > > I haven't heard anyone reporting issues with them on Xen on Arm. So I > assume that either it works or no-one is using them. > > My main point is we need to care about the common use case. 32-bit DMA > device is still a thing and caused trouble to some of our users (e.g. NXP). > > If tomorrow, someone report issue with 16-bit DMA device, then we can > consider our options how to handle. > > >>>> So I would explore to remove the NUMA check for drop the DMA zone. > FAOD, > >>>> both suggestion are for Arm only. For x86, they need to be kept. > >>>> > >>> > >>> Without introducing new flag, such as lowmem_for_dma, it's a little > >>> hard to skip the numa node check. Unless we crudely add #ifdef ARCH to > >>> common code, which is not what we want to see ... > >>> if ( !dma_bitsize && (num_online_nodes() > 1) ) > >>> dma_bitsize = arch_get_dma_bitsize(); > >> > >> ... Why do you think we need this check on Arm when NUMA is enabled? > >> > > > > I didn't think Arm needs, what I said is introduce a flag to disable > > this check for Arm or other Architectures that they don't need this > check. > > > >> We can discuss how to remove it once this is answered. > >> > > > > I think we can start to discuss it. > > How about replacing the second part of the check with a new helper > arch_have_default_dma_zone() (or a different name)?
This seems like a method worth trying, I will try to follow this way to compose next version. > > Cheers, > > -- > Julien Grall
