Re: [PATCH 2/2] qemu: Do not Use canonical path for system memory

On Tue, 12 Jan 2021 09:29:50 +0100 Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: > In commit v6.9.0-rc1~450 I've adapted libvirt to QEMU's deprecation of > -mem-path and -mem-prealloc and switched to memory-backend-* even for > system memory. My claim was that that's what QEMU does under the hood > anyway. And indeed it was: see QEMU commit v5.0.0-rc0~75^2~1^2~76 and > look at function create_default_memdev(). > > However, then commit v5.0.0-rc1~11^2~3 was merged into QEMU. While it > was fixing a bug, it also changed the create_default_memdev() function > in which it started turning off use of canonical path (by setting > "x-use-canonical-path-for-ramblock-id" attribute to false). This wasn't > documented until QEMU commit XXX. The path affects migration - the same > path has to be used on the source and on the destination. Therefore, if > there is old guest started with '-m X' it has "pc.ram" block which > doesn't use canonical path and thus when migrating to newer QEMU which > uses memory-backend-* we have to turn off the canonical path explicitly. > Otherwise, "/objects/pc.ram" path would be expected by QEMU which > doesn't match the source. > > Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1912201 > Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; > --- > > I'll replace both occurrences of 'QEMU commit XXX' once QEMU patch is > merged. > > src/qemu/qemu_command.c | 30 ++++++++++++++++--- > src/qemu/qemu_command.h | 3 +- > src/qemu/qemu_hotplug.c | 2 +- > .../hugepages-memaccess3.x86_64-latest.args | 4 +-- > 4 files changed, 31 insertions(+), 8 deletions(-) > > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c > index 6f970a3128..b99d4e5faf 100644 > --- a/src/qemu/qemu_command.c > +++ b/src/qemu/qemu_command.c

> @@ -3122,10 +3127,27 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps, > > if (qemuBuildMemoryBackendPropsShare(props, memAccess) < 0) > return -1; > + > + if (systemMemory) > + useCanonicalPath = false; > + > } else { > backendType = "memory-backend-ram"; > } > > + /* This is a terrible hack, but unfortunately there is no better way. > + * The replacement for '-m X' argument is not simple '-machine > + * memory-backend' and '-object memory-backend-*,size=X' (which was the > + * idea). This is because of create_default_memdev() in QEMU sets > + * 'x-use-canonical-path-for-ramblock-id' attribute to false and is > + * documented in QEMU in qemu-options.hx under 'memory-backend'. > + * See QEMU commit XXX. > + */ > + if (!useCanonicalPath && > + virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_X_USE_CANONICAL_PATH_FOR_RAMBLOCK_ID) && > + virJSONValueObjectAdd(props, "b:x-use-canonical-path-for-ramblock-id", false, NULL) < 0) > + return -1; is it possible to do it only for old machine types <= 4.0, to limit hack exposure?