qemu: Do not Use canonical path for system memory

In commit 88957116c9 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
8db0b20415c129cf5e577a593a4a0372d90b7cc9. 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@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_command.c

@@ -2975,7 +2975,8 @@ 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;
@@ -2992,6 +2993,10 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps,
     bool needHugepage = !!pagesize;
     bool useHugepage = !!pagesize;
     int discard = mem->discard;
+    bool disableCanonicalPath = false;
+
+    /* Disabling canonical path is required for migration compatibility of
+     * system memory objects, see below */
 
     /* The difference between @needHugepage and @useHugepage is that the latter
      * is true whenever huge page is defined for the current memory cell.
@@ -3106,6 +3111,9 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps,
         if (qemuBuildMemoryBackendPropsShare(props, memAccess) < 0)
             return -1;
 
+        if (systemMemory)
+            disableCanonicalPath = true;
+
     } else if (useHugepage || mem->nvdimmPath || memAccess ||
         def->mem.source == VIR_DOMAIN_MEMORY_SOURCE_FILE) {
 
@@ -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;
+
     if (!priv->memPrealloc &&
         virJSONValueObjectAdd(props, "B:prealloc", prealloc, NULL) < 0)
         return -1;
@@ -3263,7 +3290,7 @@ qemuBuildMemoryCellBackendStr(virDomainDefPtr def,
     mem.info.alias = alias;
 
     if ((rc = qemuBuildMemoryBackendProps(&props, alias, cfg,
-                                          priv, def, &mem, false)) < 0)
+                                          priv, def, &mem, false, false)) < 0)
         return -1;
 
     if (virQEMUBuildObjectCommandlineFromJSON(buf, props) < 0)
@@ -3292,7 +3319,7 @@ qemuBuildMemoryDimmBackendStr(virBufferPtr buf,
     alias = g_strdup_printf("mem%s", mem->info.alias);
 
     if (qemuBuildMemoryBackendProps(&props, alias, cfg,
-                                    priv, def, mem, true) < 0)
+                                    priv, def, mem, true, false) < 0)
         return -1;
 
     if (virQEMUBuildObjectCommandlineFromJSON(buf, props) < 0)
@@ -7105,7 +7132,7 @@ qemuBuildMemCommandLineMemoryDefaultBackend(virCommandPtr cmd,
     mem.info.alias = (char *) defaultRAMid;
 
     if (qemuBuildMemoryBackendProps(&props, defaultRAMid, cfg,
-                                    priv, def, &mem, false) < 0)
+                                    priv, def, &mem, false, true) < 0)
         return -1;
 
     if (virQEMUBuildObjectCommandlineFromJSON(&buf, props) < 0)

src/qemu/qemu_command.h

@@ -161,7 +161,8 @@ int qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps,
                                 qemuDomainObjPrivatePtr priv,
                                 const virDomainDef *def,
                                 const virDomainMemoryDef *mem,
-                                bool force);
+                                bool force,
+                                bool systemMemory);
 
 char *
 qemuBuildMemoryDeviceStr(const virDomainDef *def,

src/qemu/qemu_hotplug.c

@@ -2430,7 +2430,7 @@ qemuDomainAttachMemory(virQEMUDriverPtr driver,
         goto cleanup;
 
     if (qemuBuildMemoryBackendProps(&props, objalias, cfg,
-                                    priv, vm->def, mem, true) < 0)
+                                    priv, vm->def, mem, true, false) < 0)
         goto cleanup;
 
     if (qemuProcessBuildDestroyMemoryPaths(driver, vm, mem, true) < 0)

tests/qemuxml2argvdata/disk-vhostuser.x86_64-latest.args

@@ -15,7 +15,8 @@ file=/tmp/lib/domain--1-QEMUGuest1/master-key.aes \
 -machine pc,accel=tcg,usb=off,dump-guest-core=off,memory-backend=pc.ram \
 -cpu qemu64 \
 -m 214 \
--object memory-backend-memfd,id=pc.ram,share=yes,size=224395264 \
+-object memory-backend-memfd,id=pc.ram,share=yes,\
+x-use-canonical-path-for-ramblock-id=no,size=224395264 \
 -overcommit mem-lock=off \
 -smp 1,sockets=1,cores=1,threads=1 \
 -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \

tests/qemuxml2argvdata/hugepages-memaccess3.x86_64-latest.args

@@ -19,8 +19,8 @@ arch-capabilities=on,ssbd=on,xsaves=on,cmp-legacy=on,amd-ssbd=on,virt-ssbd=on,\
 rdctl-no=on,skip-l1dfl-vmentry=on,mds-no=on,pschange-mc-no=on \
 -m 4096 \
 -object memory-backend-file,id=pc.ram,\
-mem-path=/dev/hugepages2M/libvirt/qemu/-1-fedora,share=yes,prealloc=yes,\
-size=4294967296 \
+mem-path=/dev/hugepages2M/libvirt/qemu/-1-fedora,share=yes,\
+x-use-canonical-path-for-ramblock-id=no,prealloc=yes,size=4294967296 \
 -overcommit mem-lock=off \
 -smp 4,sockets=4,cores=1,threads=1 \
 -uuid 63840878-0deb-4095-97e6-fc444d9bc9fa \