Re: [PATCH v3 2/2] qemu: Do not Use canonical path for system memory

In commit 88957116c9d3cb4705380c3702c9d4315fb500bb 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 900c0ba373aada4c13d47d95330aa72ec4067ba5 and look at function create_default_memdev(). However, then commit d96c4d5f193e0e45beec80a6277728b32875bddb 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. Ideally, we would need to set it only for some machine types (4.0 and older) because newer machine types already do what we are doing. However, we treat machine types as opaque strings and therefore we don't want to parse nor inspect their versions. But then again, newer machine types already do what we are doing in this commit, so when old machine types are deprecated and removed we can remove our hack and forget it ever happened. Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1912201 Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; --- src/qemu/qemu_command.c | 35 ++++++++++++++++--- src/qemu/qemu_command.h | 3 +- src/qemu/qemu_hotplug.c | 2 +- .../disk-vhostuser.x86_64-latest.args | 3 +- .../hugepages-memaccess3.x86_64-latest.args | 4 +-- 5 files changed, 38 insertions(+), 9 deletions(-) diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c index 92036d26c0..699d89de1d 100644 --- a/src/qemu/qemu_command.c +++ b/src/qemu/qemu_command.c

@@ -3148,10 +3156,29 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps, if (qemuBuildMemoryBackendPropsShare(props, memAccess) < 0) return -1; + + if (systemMemory) + disableCanonicalPath = true; + } 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'. Note + * that QEMU consideres 'x-use-canonical-path-for-ramblock-id' stable + * and supported despite the 'x-' prefix. + * See QEMU commit 8db0b20415c129cf5e577a593a4a0372d90b7cc9. + */ + if (disableCanonicalPath && + 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;

From my side: