On Fri, Jul 04, 2025 at 07:07:53PM +0200, Ján Tomko via Devel wrote: > From: Ján Tomko <jto...@redhat.com> > > For any vhost-user device to work, the memory needs to be marked as > shared. > > What we recommend to users (e.g. in the virtiofs quide [0]) is: > <memoryBacking> > <source type='memfd'/> > <access mode='shared'/> > </memoryBacking> > > Technically, only the access mode is needed: > <memoryBacking> > <access mode='shared'/> > </memoryBacking> > But this results in libvirt assuming the file backend. > > Switch this case to use memfd which should offer better performance, > and less disk usage, at the cost of eating unswappable RAM.
The latter is a pretty significant cost, that I think has a non-trivial risk of creating deployment regressions for apps. It is not uncommon for mgmt apps to over-commit RAM, and if we're switching to a backend that's unswappable, this over-commit will now be more likely trigger the OOM killer. I don't think we can take that risk with a change like this. Any app that supports virtiofs will long have known the need to add memfd, so I don't think we win much by changing the default for that use case. > > [0] https://libvirt.org/kbase/virtiofs.html > > Signed-off-by: Ján Tomko <jto...@redhat.com> > --- > src/qemu/qemu_command.c | 9 +++++++-- > tests/qemuxmlconfdata/disk-vhostvdpa.x86_64-latest.args | 2 +- > .../net-vhostuser-multiq.x86_64-latest.args | 2 +- > .../net-vhostuser-passt.x86_64-latest.args | 2 +- > tests/qemuxmlconfdata/net-vhostuser.x86_64-latest.args | 2 +- > .../schema-reorder-domain-subelements.x86_64-latest.args | 2 +- > .../qemuxmlconfdata/vhost-user-fs-ccw.s390x-latest.args | 2 +- > 7 files changed, 13 insertions(+), 8 deletions(-) > > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c > index 7658cc4d39..ca9d4d6609 100644 > --- a/src/qemu/qemu_command.c > +++ b/src/qemu/qemu_command.c > @@ -3417,8 +3417,13 @@ qemuBuildMemoryBackendProps(virJSONValue > **backendProps, > if (!priv->memPrealloc) > prealloc = true; > > - } else if (!nvdimmPath && > - def->mem.source == VIR_DOMAIN_MEMORY_SOURCE_MEMFD) { > + } else if ((!nvdimmPath && > + def->mem.source == VIR_DOMAIN_MEMORY_SOURCE_MEMFD) || > + (systemMemory && > + memAccess && > + !useHugepage && > + def->mem.source == VIR_DOMAIN_MEMORY_SOURCE_NONE && > + virQEMUCapsGet(priv->qemuCaps, > QEMU_CAPS_OBJECT_MEMORY_MEMFD))) { > backendType = "memory-backend-memfd"; > > if (useHugepage && > diff --git a/tests/qemuxmlconfdata/disk-vhostvdpa.x86_64-latest.args > b/tests/qemuxmlconfdata/disk-vhostvdpa.x86_64-latest.args > index 93d2ef0d98..04732df68a 100644 > --- a/tests/qemuxmlconfdata/disk-vhostvdpa.x86_64-latest.args > +++ b/tests/qemuxmlconfdata/disk-vhostvdpa.x86_64-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \ > -accel tcg \ > -cpu qemu64 \ > -m size=219136k \ > --object > '{"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-QEMUGuest1/pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > -overcommit mem-lock=off \ > -smp 1,sockets=1,cores=1,threads=1 \ > -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \ > diff --git a/tests/qemuxmlconfdata/net-vhostuser-multiq.x86_64-latest.args > b/tests/qemuxmlconfdata/net-vhostuser-multiq.x86_64-latest.args > index 4ea3d4eebd..ac5c8b7652 100644 > --- a/tests/qemuxmlconfdata/net-vhostuser-multiq.x86_64-latest.args > +++ b/tests/qemuxmlconfdata/net-vhostuser-multiq.x86_64-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \ > -accel tcg \ > -cpu qemu64 \ > -m size=219136k \ > --object > '{"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-QEMUGuest1/pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > -overcommit mem-lock=off \ > -smp 1,sockets=1,cores=1,threads=1 \ > -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \ > diff --git a/tests/qemuxmlconfdata/net-vhostuser-passt.x86_64-latest.args > b/tests/qemuxmlconfdata/net-vhostuser-passt.x86_64-latest.args > index afbbe188cf..ee395deffe 100644 > --- a/tests/qemuxmlconfdata/net-vhostuser-passt.x86_64-latest.args > +++ b/tests/qemuxmlconfdata/net-vhostuser-passt.x86_64-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \ > -accel tcg \ > -cpu qemu64 \ > -m size=219136k \ > --object > '{"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-QEMUGuest1/pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > -overcommit mem-lock=off \ > -smp 1,sockets=1,cores=1,threads=1 \ > -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \ > diff --git a/tests/qemuxmlconfdata/net-vhostuser.x86_64-latest.args > b/tests/qemuxmlconfdata/net-vhostuser.x86_64-latest.args > index f5925c77fe..74454d3d24 100644 > --- a/tests/qemuxmlconfdata/net-vhostuser.x86_64-latest.args > +++ b/tests/qemuxmlconfdata/net-vhostuser.x86_64-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \ > -accel tcg \ > -cpu qemu64 \ > -m size=219136k \ > --object > '{"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-QEMUGuest1/pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > -overcommit mem-lock=off \ > -smp 1,sockets=1,cores=1,threads=1 \ > -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \ > diff --git > a/tests/qemuxmlconfdata/schema-reorder-domain-subelements.x86_64-latest.args > b/tests/qemuxmlconfdata/schema-reorder-domain-subelements.x86_64-latest.args > index 76df9c30b0..5fb594df87 100644 > --- > a/tests/qemuxmlconfdata/schema-reorder-domain-subelements.x86_64-latest.args > +++ > b/tests/qemuxmlconfdata/schema-reorder-domain-subelements.x86_64-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-passtvhostuu/.config \ > -accel kvm \ > -cpu qemu64 \ > -m size=16777216k \ > --object > '{"qom-type":"memory-backend-file","id":"pc.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-passtvhostuu/pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":17179869184}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"pc.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":17179869184}' > \ > -overcommit mem-lock=off \ > -smp 16,sockets=16,cores=1,threads=1 \ > -uuid d79698e0-46b2-4459-87ee-7c762990dd6e \ > diff --git a/tests/qemuxmlconfdata/vhost-user-fs-ccw.s390x-latest.args > b/tests/qemuxmlconfdata/vhost-user-fs-ccw.s390x-latest.args > index 72ca38c305..eed9af8bb1 100644 > --- a/tests/qemuxmlconfdata/vhost-user-fs-ccw.s390x-latest.args > +++ b/tests/qemuxmlconfdata/vhost-user-fs-ccw.s390x-latest.args > @@ -14,7 +14,7 @@ > XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \ > -accel tcg \ > -cpu qemu \ > -m size=219136k \ > --object > '{"qom-type":"memory-backend-file","id":"s390.ram","mem-path":"/var/lib/libvirt/qemu/ram/-1-QEMUGuest1/s390.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > +-object > '{"qom-type":"memory-backend-memfd","id":"s390.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":224395264}' > \ > -overcommit mem-lock=off \ > -smp 1,sockets=1,cores=1,threads=1 \ > -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \ > -- > 2.50.0 > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
