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Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backe
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram |
Date: |
Thu, 1 Feb 2018 04:22:07 +0200 |
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
[Qemu-devel] [PATCH V8 2/4] docs: add pvrdma device documentation., Marcel Apfelbaum, 2018/01/17
[Qemu-devel] [PATCH V8 4/4] MAINTAINERS: add entry for hw/rdma, Marcel Apfelbaum, 2018/01/17
[Qemu-devel] [PATCH V8 3/4] pvrdma: initial implementation, Marcel Apfelbaum, 2018/01/17
Re: [Qemu-devel] [PATCH V8 0/4] hw/pvrdma: PVRDMA device implementation, no-reply, 2018/01/17