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

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

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.


>  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.

This will fix rdma with hugetlbfs as well which is currently broken.


> -- 
> Eduardo