Re: [PATCH v2 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. 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 | 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 @@ -2950,7 +2950,8 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps, qemuDomainObjPrivatePtr priv, const virDomainDef *def, const virDomainMemoryDef *mem, - bool force) + bool force, + bool systemMemory) { const char *backendType = "memory-backend-file"; virDomainNumatuneMemMode mode; @@ -2967,6 +2968,7 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps, bool needHugepage = !!pagesize; bool useHugepage = !!pagesize; int discard = mem->discard; + bool useCanonicalPath = true;

@@ -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)