Re: [libvirt] [Qemu-devel] [PATCH] deprecate -mem-path fallback to anonymous RAM

Igor Mammedov <imammedo(a)redhat.com&gt; writes: > On Mon, 24 Jun 2019 10:17:33 +0200 > Markus Armbruster <armbru(a)redhat.com&gt; wrote: > >> Igor Mammedov <imammedo(a)redhat.com&gt; writes: >> >> > Fallback might affect guest or worse whole host performance >> > or functionality if backing file were used to share guest RAM >> > with another process. >> > >> > Patch deprecates fallback so that we could remove it in future >> > and ensure that QEMU will provide expected behavior and fail if >> > it can't use user provided backing file. >> > >> > Signed-off-by: Igor Mammedov <imammedo(a)redhat.com&gt; >> > --- >> > PS: >> > Patch is written on top of >> > [PATCH v4 0/3] numa: deprecate '-numa node, mem' and default memory distribution >> > to avoid conflicts in qemu-deprecated.texi >> > >> > numa.c | 4 ++-- >> > qemu-deprecated.texi | 8 ++++++++ >> > 2 files changed, 10 insertions(+), 2 deletions(-) >> > >> > diff --git a/numa.c b/numa.c >> > index 91a29138a2..53d67b8ad9 100644 >> > --- a/numa.c >> > +++ b/numa.c >> > @@ -494,8 +494,8 @@ static void allocate_system_memory_nonnuma(MemoryRegion *mr, Object *owner, >> > if (mem_prealloc) { >> > exit(1); >> > } >> > - error_report("falling back to regular RAM allocation."); >> > - >> > + warn_report("falling back to regular RAM allocation. " >> > + "Fallback to RAM allocation is deprecated."); >> >> Can we give the user clues on how to avoid the deprecated fallback? > > I've intentionally left it out for a lack of clear enough advise. > Something like: > "Make sure that host has resources to map file pointed by -mem-path" > would be pretty useless. I see. > I think describing how host should be configured in various ways > depending on type of backing storage is well out of scope of any > QEMU documentation. But if you have an idea to what to put there > (or what to put in deprecation doc and refer to from here), > I'll add it on respin. > >> Warning message nitpick: the message should be a single phrase, with no >> newline or trailing punctuation. Suggest something like >> >> warn_report("falling back to regular RAM allocation"); >> error_printf("This is deprecated. <Advice on what\n" >> "to do goes here>\n"); >> >> > /* Legacy behavior: if allocation failed, fall back to >> > * regular RAM allocation. >> > */ >> > diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi >> > index 2fe9b72121..2193705644 100644 >> > --- a/qemu-deprecated.texi >> > +++ b/qemu-deprecated.texi >> > @@ -112,6 +112,14 @@ QEMU using implicit generic or board specific splitting rule. >> > Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if >> > it's supported by used machine type) to define mapping explictly instead. >> > >> > +@subsection -mem-path fallback to RAM (since 4.1) >> > +Currently if system memory allocation from file pointed by @option{mem-path} >> > +fails, QEMU fallbacks to allocating from anonymous RAM. Which might result >> > +in unpredictable behavior since provided backing file wasn't used. >> >> >> Noch such verb "to fallback", obvious fix "QEMU falls back to" >> >> Suggest "RAM, which might". >> >> Better: "since the backing file specified by the user is ignored". >> >> > In future >> > +QEMU will not fallback and fail to start up, so user could fix his/her QEMU/host >> > +configuration or explicitly use -m without -mem-path if system memory allocated >> > +from anonymous RAM suits usecase. >> >> What's "system memory allocation"? > Using man page language, would be > 'guest startup RAM size' > acceptable? > > >> Perhaps: "In the future, QEMU will not fall back, but fail instead. >> Adjust either the host configuration [FIXME how?] or the QEMU >> configuration [FIXME how?]." > > Maybe > " > In the future, QEMU will not fall back, but fail instead. > Adjust either the QEMU configuration by removing @option{-mem-path} so > QEMU will use only anonymous or host configuration to make sure that Do you mean "only anonymous memory"?

> there are sufficient resources on backing storage pointed by -mem-path > to allocate amount specified by @option{-m}. > " Perhaps this could suffice: "Users will be responsible for making sure the backing storage specified with -mem-path can actually provide the guest RAM configured with -m."

>> > + >> > @section QEMU Machine Protocol (QMP) commands >> > >> > @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0) >>