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Re: [Qemu-devel] [PATCH v5 5/6] Add memfd based hostmem
From: |
Marc-André Lureau |
Subject: |
Re: [Qemu-devel] [PATCH v5 5/6] Add memfd based hostmem |
Date: |
Mon, 23 Oct 2017 07:41:57 -0400 (EDT) |
----- Original Message -----
> On Mon, Oct 23, 2017 at 10:59:09AM +0100, Marc-André Lureau wrote:
> > Add a new memory backend, similar to hostmem-file, except that it
> > doesn't need to create files. It also enforces memory sealing.
> >
> > This backend is mainly useful for sharing the memory with other
> > processes.
> >
> > Note that Linux supports transparent huge-pages of shmem/memfd memory
> > since 4.8. It is relatively easier to set up THP than a dedicate
> > hugepage mount point by using "madvise" in
> > /sys/kernel/mm/transparent_hugepage/shmem_enabled.
> >
> > Since 4.14, memfd allows to set hugetlb requirement explicitly.
> >
> > Usage:
> > -object memory-backend-memfd,id=mem1,size=1G
>
> [snip]
>
> > +static void
> > +memfd_backend_class_init(ObjectClass *oc, void *data)
> > +{
> > + HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
> > +
> > + bc->alloc = memfd_backend_memory_alloc;
> > +
> > + object_class_property_add_bool(oc, "hugetlb",
> > + memfd_backend_get_hugetlb,
> > + memfd_backend_set_hugetlb,
> > + &error_abort);
>
> I tend to think that instead of a bool hugetlb, we should take an
> integer page size instead eg hugepagesize=2M instead of hugetlb=true
>
Well, how would you then create a memfd without explicit hugetlb request or one
with default hugetlb size?
I think size should be a different property. I'll add it.
>
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 3728e9b4dd..5828caefeb 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -4208,6 +4208,21 @@ that @option{discard-data} is only an optimization,
> > and QEMU
> > might not discard file contents if it aborts unexpectedly or is
> > terminated using SIGKILL.
> >
> > address@hidden -object
> > memory-backend-memfd,address@hidden,address@hidden,address@hidden|off},address@hidden|off}
> > +
> > +Creates an anonymous memory file backend object, which allows QEMU to
> > +share the memory with an external process in some cases (e.g. when
> > +using vhost-user). The memory is allocated with memfd and optional
> > +sealing. (Linux only)
> > +
> > +The @option{id} parameter is a unique ID that will be used to
> > +reference this memory region when configuring the @option{-numa}
> > +argument. The @option{size} option provides the size of the memory
> > +region, and accepts common suffixes, eg @option{500M}. The
> > address@hidden option creates a sealed-file, that will block further
> > +resizing the memory ('on' by default). The @option{hugetlb} option
> > +specify the file to be created resides in the hugetlbfs filesystem.
>
> This should document that you can't combine sealing and huge pages
>
Ok, I am wondering if this is a temporary limitation, I have asked the patch
author about it (it looks possible to me, I'll eventually try implementing it)
[Qemu-devel] [PATCH v5 6/6] tests: use memfd in vhost-user-test, Marc-André Lureau, 2017/10/23