Re: [libvirt] QEMU capabilities vs machine types
On 11.02.2015 16:47, Daniel P. Berrange wrote:
On Wed, Feb 11, 2015 at 04:31:53PM +0100, Michal Privoznik wrote:
>
There are two reasons why we query & check the supported capabilities
from QEMU
1. There are multiple possible CLI args for the same feature and
we need to choose the "best" one to use
2. The feature is not supported and we want to give the caller a
better error message than they'd get from QEMU
I'm unclear from the bug which scenario applies here.
If it is scenario 2 though, I'd just mark it as CANTFIX or WONTFIX,
as no matter what we do the user would get an error. It is not worth
making our capability matrix a factor of 10+ bigger just to get a
better error message.
If it is scenario 1, I think the burden is on QEMU to solve. The
memory-backend-{file,ram} CLI flags shouldn't be tied to guest
machine types, as they are backend config setup options that should
not impact guest ABI.
It's somewhere in between 1 and 2. Back in RHEL-7.0 days libvirt would
have created a guest with:
-numa node,...,mem=1337
But if qemu reports it support memory-backend-ram, libvirt tries to use it:
-object memory-backend-ram,id=ram-node0,size=1337M,... \
-numa node,...,memdev=ram-node0
This breaks migration to newer qemu which is in RHEL-7.1. If qemu would
report the correct value, we can generate the correct command line and
migration succeeds. However, our fault is, we are not asking the correct
question anyway.
Michal