Hi Julien, > -----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 : ) Anyway, keep the arch_get_dma_bitsize as original maybe the best approach for now. > > > > > >> Anyway, there are plenty of platform out that have devices which can't > >> DMA into memory above 32-bit. On RPI, this is even lower (30-bit). > >> > >> So I would be cautious to change the default limit. > >> > > > > How about return 0 when platform doesn't specify the limit? > > In my opinion, arbitrary to give 32 on AArch64 doesn't make sense. > We have to care about the common use-case. We added this restriction a > few years ago because we had a few reports of users using only 32-bit > DMA capable device. > > > But as you mentioned, if Xen is running on a platform with DMA > limitation, > > but we have not listed this platform in Xen supported list, Xen cannot > > get DMA limit from platform->dma_bitsize. In this case, return 0 will > > also cause some issue. > > > >> At the moment, the only place on Arm where we need DMA-able memory is > >> for dom0. This is allocated at boot and can't change afterwards (for > now). > >> > > > > For Dom0, we squash the patch#5 into this patch? > > Let me answer with another question. Why should we modify the Arm code > rather than the common code? IOW... As I answered above, let's keep arch_get_dma_bitsize as original then we don't need to modify Arm code. > > > > >> 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. > Cheers, > > -- > Julien Grall
