On Mon, Nov 30, 2020 at 11:06:14 +0100, Michal Privoznik wrote:
Currently, we configure QEMU to prealloc memory almost by default. Well, by default for NVDIMMs, hugepages and if user asked us to (via memoryBacking <allocation mode="immediate"/>).
However, there are two cases where this approach is not the best:
1) in case when guest's NVDIMM is backed by real life NVDIMM. In this case users should put <pmem/> into the <memory/> device <source/>, like this:
<memory model='nvdimm' access='shared'> <source> <path>/dev/pmem0</path> <pmem/> </source> </memory>
Instructing QEMU to do prealloc in this case means that each page of the NVDIMM is "touched" (the first byte is read and written back - see QEMU commit v2.9.0-rc1~26^2) which cripples device wear.
2) if free-page-reporting is turned on. While the free-page-reporting feature might not have a catchy or obvious name, when enabled it instructs KVM and subsequently QEMU to free pages no longer used by guest resulting in smaller memory footprint. And preallocating whole memory goes against this.
The BZ comment 11 mentions another, third case 'virtio-mem' but that is not implemented in libvirt, yet.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1894053 Signed-off-by: Michal Privoznik <mprivozn@redhat.com> --- src/qemu/qemu_command.c | 11 +++++++++-- .../memory-hotplug-nvdimm-pmem.x86_64-latest.args | 2 +- 2 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c index 479bcc0b0c..3df8b5ac76 100644 --- a/src/qemu/qemu_command.c +++ b/src/qemu/qemu_command.c @@ -2977,7 +2977,11 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps, if (discard == VIR_TRISTATE_BOOL_ABSENT) discard = def->mem.discard;
- if (def->mem.allocation == VIR_DOMAIN_MEMORY_ALLOCATION_IMMEDIATE) + /* The whole point of free_page_reporting is that as soon as guest frees + * any memory it is freed in the host too. Prealloc doesn't make much sense + * then. */ + if (def->mem.allocation == VIR_DOMAIN_MEMORY_ALLOCATION_IMMEDIATE && + def->memballoon->free_page_reporting != VIR_TRISTATE_SWITCH_ON) prealloc = true;
IIUC def->mem.allocation is an explicit user-specified configuration, in such case we should not override the user wish based on a different configuration. If they don't make sense together technically, we should reject the config rather than silently changing it. If it's just semantically wrong and pre-existing we may leave it be. Additionally the patch is missing a test case for this scenario.