> -----Original Message----- > From: Tian, Kevin [mailto:kevin.tian@xxxxxxxxx] > Sent: Tuesday, February 2, 2021 3:52 PM > To: Jason Gunthorpe <jgg@xxxxxxxx> > Cc: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; chensihang (A) > <chensihang1@xxxxxxxxxxxxx>; Arnd Bergmann <arnd@xxxxxxxx>; Greg > Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; Zhangfei Gao > <zhangfei.gao@xxxxxxxxxx>; Liguozhu (Kenneth) <liguozhu@xxxxxxxxxxxxx>; > linux-accelerators@xxxxxxxxxxxxxxxx > Subject: RE: [RFC PATCH v2] uacce: Add uacce_ctrl misc device > > > From: Jason Gunthorpe <jgg@xxxxxxxx> > > Sent: Tuesday, February 2, 2021 7:44 AM > > > > On Fri, Jan 29, 2021 at 10:09:03AM +0000, Tian, Kevin wrote: > > > > SVA is not doom to work with IO page fault only. If we have SVA+pin, > > > > we would get both sharing address and stable I/O latency. > > > > > > Isn't it like a traditional MAP_DMA API (imply pinning) plus specifying > > > cpu_va of the memory pool as the iova? > > > > I think their issue is the HW can't do the cpu_va trick without also > > involving the system IOMMU in a SVA mode > > > > This is the part that I didn't understand. Using cpu_va in a MAP_DMA > interface doesn't require device support. It's just an user-specified > address to be mapped into the IOMMU page table. On the other hand, The background is that uacce is based on SVA and we are building applications on uacce: https://www.kernel.org/doc/html/v5.10/misc-devices/uacce.html so IOMMU simply uses the page table of MMU, and don't do any special mapping to an user-specified address. We don't break the basic assumption that uacce is using SVA, otherwise, we need to re-build uacce and the whole base. > sharing CPU page table through a SVA interface for an usage where I/O > page faults must be completely avoided seems a misleading attempt. That is not for completely avoiding IO page fault, that is just an extension for high-performance I/O case, providing a way to avoid IO latency jitter. Using it or not is totally up to users. > Even if people do want this model (e.g. mix pinning+fault), it should be > a mm syscall as Greg pointed out, not specific to sva. > We are glad to make it a syscall if people are happy with it. The simplest way would be a syscall similar with userfaultfd if we don't want to mess up mm_struct. > Thanks > Kevin Thanks Barry
