Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram

["Michael S. Tsirkin" <mst@redhat.com>]

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.

To be more precise, it's the guest supplied IOVA that is sent to the card.

> > 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.
> 
> >>
> >> An alternative would be 0-based MR, QEMU intercepts the post-send
> >> operations and can substract the guest VA base address.
> >> However I didn't see the implementation in kernel for 0 based MRs
> >> and also the RDMA maintainer said it would work for local keys
> >> and not for remote keys.
> > 
> > This is also unexpected: are GVAs visible to the virtual RDMA
> > hardware? 
> 
> Yes, explained above
> 
> > Where does the QEMU pvrdma code translates GVAs to
> > GPAs?
> > 
> 
> During reg_mr (memory registration commands)
> Then it registers the same addresses to the real HW.
> (as Host virtual addresses)
> 
> Thanks,
> Marcel


The full fix would be to allow QEMU to map a list of
pages to a guest supplied IOVA.

> >>
> >>> This will fix rdma with hugetlbfs as well which is currently broken.
> >>>
> >>>
> >>
> >> There is already a discussion on the linux-rdma list:
> >>     https://www.spinics.net/lists/linux-rdma/msg60079.html
> >> But it will take some (actually a lot of) time, we are currently talking about
> >> a possible API. And it does not solve the re-mapping...
> >>
> >> Thanks,
> >> Marcel
> >>
> >>>> -- 
> >>>> Eduardo
> >>
> >