> -----Original Message----- > From: Tian, Kevin [mailto:kevin.tian@xxxxxxxxx] > Sent: Friday, January 29, 2021 11:09 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Jason Gunthorpe > <jgg@xxxxxxxx> > Cc: 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: Song Bao Hua (Barry Song) > > Sent: Tuesday, January 26, 2021 9:27 AM > > > > > -----Original Message----- > > > From: Jason Gunthorpe [mailto:jgg@xxxxxxxx] > > > Sent: Tuesday, January 26, 2021 2:13 PM > > > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx> > > > Cc: Wangzhou (B) <wangzhou1@xxxxxxxxxxxxx>; Greg Kroah-Hartman > > > <gregkh@xxxxxxxxxxxxxxxxxxx>; Arnd Bergmann <arnd@xxxxxxxx>; > > Zhangfei Gao > > > <zhangfei.gao@xxxxxxxxxx>; linux-accelerators@xxxxxxxxxxxxxxxx; > > > linux-kernel@xxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; > > > linux-mm@xxxxxxxxx; Liguozhu (Kenneth) <liguozhu@xxxxxxxxxxxxx>; > > chensihang > > > (A) <chensihang1@xxxxxxxxxxxxx> > > > Subject: Re: [RFC PATCH v2] uacce: Add uacce_ctrl misc device > > > > > > On Mon, Jan 25, 2021 at 11:35:22PM +0000, Song Bao Hua (Barry Song) > > wrote: > > > > > > > > On Mon, Jan 25, 2021 at 10:21:14PM +0000, Song Bao Hua (Barry Song) > > wrote: > > > > > > mlock, while certainly be able to prevent swapping out, it won't > > > > > > be able to stop page moving due to: > > > > > > * memory compaction in alloc_pages() > > > > > > * making huge pages > > > > > > * numa balance > > > > > > * memory compaction in CMA > > > > > > > > > > Enabling those things is a major reason to have SVA device in the > > > > > first place, providing a SW API to turn it all off seems like the > > > > > wrong direction. > > > > > > > > I wouldn't say this is a major reason to have SVA. If we read the > > > > history of SVA and papers, people would think easy programming due > > > > to data struct sharing between cpu and device, and process space > > > > isolation in device would be the major reasons for SVA. SVA also > > > > declares it supports zero-copy while zero-copy doesn't necessarily > > > > depend on SVA. > > > > > > Once you have to explicitly make system calls to declare memory under > > > IO, you loose all of that. > > > > > > Since you've asked the app to be explicit about the DMAs it intends to > > > do, there is not really much reason to use SVA for those DMAs anymore. > > > > Let's see a non-SVA case. We are not using SVA, we can have > > a memory pool by hugetlb or pin, and app can allocate memory > > from this pool, and get stable I/O performance on the memory > > from the pool. But device has its separate page table which > > is not bound with this process, thus lacking the protection > > of process space isolation. Plus, CPU and device are using > > different address. > > > > And then we move to SVA case, we can still have a memory pool > > by hugetlb or pin, and app can allocate memory from this pool > > since this pool is mapped to the address space of the process, > > and we are able to get stable I/O performance since it is always > > there. But in this case, device is using the page table of > > process with the full permission control. > > And they are using same address and can possibly enjoy the easy > > programming if HW supports. > > > > 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 it enjoys the advantage of stable I/O latency of traditional MAP_DMA, and also uses the process page table which SVA can provide. The major difference is that in SVA case, iova totally belongs to process and is as normal as other heap/stack/data: p = mmap(.....MAP_ANON....); ioctl(/dev/acc, p, PIN); SVA for itself, provides the ability to guarantee the address space isolation of multiple processes. If the device can access the data struct such as list, tree directly, they can further enjoy the convenience of programming SVA gives. So we are looking for a combination of stable io latency of traditional DMA map and the ability of SVA. > > Thanks > Kevin Thanks Barry
