On 01/02/2018 20:51, Eduardo Habkost wrote: > On Thu, Feb 01, 2018 at 08:31:09PM +0200, Marcel Apfelbaum wrote: >> On 01/02/2018 20:21, Eduardo Habkost wrote: >>> On Thu, Feb 01, 2018 at 08:03:53PM +0200, Marcel Apfelbaum wrote: >>>> On 01/02/2018 15:53, Eduardo Habkost wrote: >>>>> On Thu, Feb 01, 2018 at 02:29:25PM +0200, Marcel Apfelbaum wrote: >>>>>> On 01/02/2018 14:10, Eduardo Habkost wrote: >>>>>>> On Thu, Feb 01, 2018 at 07:36:50AM +0200, Marcel Apfelbaum wrote: >>>>>>>> On 01/02/2018 4:22, Michael S. Tsirkin wrote: >>>>>>>>> On Wed, Jan 31, 2018 at 09:34:22PM -0200, Eduardo Habkost wrote: >>>>>>> [...] >>>>>>>>>> BTW, what's the root cause for requiring HVAs in the buffer? >>>>>>>>> >>>>>>>>> It's a side effect of the kernel/userspace API which always wants >>>>>>>>> a single HVA/len pair to map memory for the application. >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> Hi Eduardo and Michael, >>>>>>>> >>>>>>>>>> Can >>>>>>>>>> this be fixed? >>>>>>>>> >>>>>>>>> I think yes. It'd need to be a kernel patch for the RDMA subsystem >>>>>>>>> mapping an s/g list with actual memory. The HVA/len pair would then >>>>>>>>> just >>>>>>>>> be used to refer to the region, without creating the two mappings. >>>>>>>>> >>>>>>>>> Something like splitting the register mr into >>>>>>>>> >>>>>>>>> mr = create mr (va/len) - allocate a handle and record the va/len >>>>>>>>> >>>>>>>>> addmemory(mr, offset, hva, len) - pin memory >>>>>>>>> >>>>>>>>> register mr - pass it to HW >>>>>>>>> >>>>>>>>> As a nice side effect we won't burn so much virtual address space. >>>>>>>>> >>>>>>>> >>>>>>>> We would still need a contiguous virtual address space range (for >>>>>>>> post-send) >>>>>>>> which we don't have since guest contiguous virtual address space >>>>>>>> will always end up as non-contiguous host virtual address space. >>>>>>>> >>>>>>>> I am not sure the RDMA HW can handle a large VA with holes. >>>>>>> >>>>>>> I'm confused. Why would the hardware see and care about virtual >>>>>>> addresses? >>>>>> >>>>>> The post-send operations bypasses the kernel, and the process >>>>>> puts in the work request GVA addresses. >>>>>> >>>>>>> How exactly does the hardware translates VAs to >>>>>>> PAs? >>>>>> >>>>>> The HW maintains a page-directory like structure different form MMU >>>>>> VA -> phys pages >>>>>> >>>>>>> What if the process page tables change? >>>>>>> >>>>>> >>>>>> Since the page tables the HW uses are their own, we just need the phys >>>>>> page to be pinned. >>>>> >>>>> So there's no hardware-imposed requirement that the hardware VAs >>>>> (mapped by the HW page directory) match the VAs in QEMU >>>>> address-space, right? >>>> >>>> Actually there is. Today it works exactly as you described. >>> >>> Are you sure there's such hardware-imposed requirement? >>> >> >> Yes. >> >>> Why would the hardware require VAs to match the ones in the >>> userspace address-space, if it doesn't use the CPU MMU at all? >>> >> >> It works like that: >> >> 1. We register a buffer from the process address space >> giving its base address and length. >> This call goes to kernel which in turn pins the phys pages >> and registers them with the device *together* with the base >> address (virtual address!) >> 2. The device builds its own page tables to be able to translate >> the virtual addresses to actual phys pages. > > How would the device be able to do that? It would require the > device to look at the process page tables, wouldn't it? Isn't > the HW IOVA->PA translation table built by the OS? >
As stated above, these are tables private for the device. (They even have a hw vendor specific layout I think, since the device holds some cache) The device looks at its own private page tables, and not to the OS ones. > >> 3. The process executes post-send requests directly to hw by-passing >> the kernel giving process virtual addresses in work requests. >> 4. The device uses its own page tables to translate the virtual >> addresses to phys pages and sending them. >> >> Theoretically is possible to send any contiguous IOVA instead of >> process's one but is not how is working today. >> >> Makes sense? >