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 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:
> >> On Wed, Jan 31, 2018 at 11:10:07PM +0200, Michael S. Tsirkin wrote:
> >>> On Wed, Jan 31, 2018 at 06:40:59PM -0200, Eduardo Habkost wrote:
> >>>> On Wed, Jan 17, 2018 at 11:54:18AM +0200, Marcel Apfelbaum wrote:
> >>>>> Currently only file backed memory backend can
> >>>>> be created with a "share" flag in order to allow
> >>>>> sharing guest RAM with other processes in the host.
> >>>>>
> >>>>> Add the "share" flag also to RAM Memory Backend
> >>>>> in order to allow remapping parts of the guest RAM
> >>>>> to different host virtual addresses. This is needed
> >>>>> by the RDMA devices in order to remap non-contiguous
> >>>>> QEMU virtual addresses to a contiguous virtual address range.
> >>>>>
> >>>>
> >>>> Why do we need to make this configurable?  Would anything break
> >>>> if MAP_SHARED was always used if possible?
> >>>
> >>> See Documentation/vm/numa_memory_policy.txt for a list
> >>> of complications.
> >>
> >> Ew.
> >>
> >>>
> >>> Maybe we should more of an effort to detect and report these
> >>> issues.
> >>
> >> Probably.  Having other features breaking silently when using
> >> pvrdma doesn't sound good.  We must at least document those
> >> problems in the documentation for memory-backend-ram.
> >>
> >> 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.

It just needs to be contiguous in the HCA virtual address space.
Software never accesses through this pointer.
In other words - basically expose register physical mr to userspace.


> 
> I am not sure the RDMA HW can handle a large VA with holes.
> 
> 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 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.

You probably need to pass the s/g piece by piece since it might exceed
any reasonable array size.

> And it does not solve the re-mapping...
> 
> Thanks,
> Marcel

Haven't read through that discussion. But at least what I posted solves
it since you do not need it contiguous in HVA any longer.

> >> -- 
> >> Eduardo