On Fri, Jun 09, 2017 at 09:58:47AM +0800, Peter Xu wrote: > On Thu, Jun 08, 2017 at 09:59:50PM +0300, Michael S. Tsirkin wrote: > > On Thu, Jun 08, 2017 at 02:11:50PM +0800, Peter Xu wrote: > > > On Wed, Jun 07, 2017 at 04:07:20PM +0300, Michael S. Tsirkin wrote: > > > > On Wed, Jun 07, 2017 at 11:44:43AM +0800, Peter Xu wrote: > > > > > On Wed, Jun 07, 2017 at 09:47:05AM +1000, David Gibson wrote: > > > > > > On Tue, Jun 06, 2017 at 04:34:30PM +0200, Paolo Bonzini wrote: > > > > > > > > > > > > > > > > > > > > > On 05/06/2017 05:07, Peter Xu wrote: > > > > > > > > I don't sure whether it'll be a good interface for IOTLB. AFAIU > > > > > > > > at > > > > > > > > least for VT-d, the IOMMU translation is page aligned which is > > > > > > > > defined > > > > > > > > by spec, so it makes sense that (again at least for VT-d) here > > > > > > > > we'd > > > > > > > > better just use page_mask/addr_mask. > > > > > > > > > > > > > > > > That's also how I know about IOMMU in general - I assume it do > > > > > > > > the > > > > > > > > translations always with page masks (never arbitary length), > > > > > > > > though > > > > > > > > page size can differ from platfrom to platform, that's why here > > > > > > > > the > > > > > > > > IOTLB interface used addr_mask, then it works for all > > > > > > > > platforms. I > > > > > > > > don't know whether I'm 100% correct here though. > > > > > > > > > > > > > > > > Maybe David/Paolo/... would comment as well? > > > > > > > > > > > > > > I would ask David. There are PowerPC MMUs that allow fast lookup > > > > > > > of > > > > > > > arbitrarily-sized windows (not necessarily power of two), > > > > > > > > > > > > Uh.. I'm not sure what you mean here. You might be thinking of the > > > > > > BATs which really old (32-bit) PowerPC MMUs had - those allow > > > > > > arbitrary large block translations, but they do have to be a power > > > > > > of > > > > > > two. > > > > > > > > > > > > > so maybe the > > > > > > > IOMMUs can do the same. > > > > > > > > > > > > The only Power IOMMU I know about uses a fixed, power-of-two page > > > > > > size > > > > > > per DMA window. > > > > > > > > > > If so, I would still be inclined to keep using masks for QEMU IOTLB. > > > > > Then, my first two patches should still stand. > > > > > > > > > > I am just afraid that not using masks will diverge the emulation from > > > > > real hardware and brings trouble one day. > > > > > > > > > > For vhost IOTLB interface, it does not need to be strictly aligned to > > > > > QEMU IOMMU IOTLB definition, and that's how it's working now (current > > > > > vhost iotlb allows arbitary length, and I think it's good). So imho we > > > > > don't really need to worry about the performance - after all, we can > > > > > do everything customized for vhost, just like what patch 3 did (yeah, > > > > > it can be better...). > > > > > > > > > > Thanks, > > > > > > > > Pre-faults is also something that does not happen on real hardware. > > > > And it's about security so a bigger issue. > > > > > > > > If I had to choose between that and using non-power-of-2 in > > > > the API, I'd go for non-power-of-2. Let backends that can only > > > > support power of 2 split it up to multiple transactions. > > > > > > The problem is that when I was fixing the problem that vhost had with > > > PT (a764040, "exec: abstract address_space_do_translate()"), I did > > > broke the IOTLB translation a bit (it was using page masks). IMHO we > > > need to fix it first for correctness (patch 1/2). > > > > > > For patch 3, if we can have Jason's patch to allow dynamic > > > iommu_platform switching, that'll be the best, then I can rewrite > > > patch 3 with the switching logic rather than caching anything. But > > > IMHO that can be separated from patch 1/2 if you like. > > > > > > Or do you have better suggestion on how should we fix it? > > > > > > Thanks, > > > > Can we drop masks completely and replace with length? I think we > > should do that instead of trying to fix masks. > > Do you mean to modify IOMMUTLBEntry.addr_mask into length? > > Again, I am not sure this is good... At least we need to get ack from > David since spapr should be the initial user of it, and possibly also > Alex since vfio should be assuming that (IIUC both in QEMU and kernel) > addr_mask is page masks rather than arbirary length.
So, I don't see that using size instead of mask would be a particular problem for spapr. However, I also don't see any advantage to switching. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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