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. > -- > Peter Xu
