Andrew Jones <andrew.jones@xxxxxxxxx> writes:
On Fri, Jan 06, 2023 at 05:37:16PM +0000, Colton Lewis wrote:
Andrew Jones <andrew.jones@xxxxxxxxx> writes:
> > We could cap at 8 for ACCEL=tcg. Even if no one cares, I'm tempted
to do
> > it so no one hits the same little landmine as me in the future.
> TCG supports up to 255 CPUs. The only reason it'd have a max of 8 is if
> you were configuring a GICv2 instead of a GICv3.
That makes sense as it was the GICv2 tests failing that led me to this
rabbit hole. In that case, it should be completely safe to delete the
loop because all the GICv2 tests have ternary condition to cap at 8
already.
How did your gicv2 tests hit the problem?
Mysteriously. QEMU/TCG failed those tests when given -smp 4 but not -smp
8. There's the unresoved question of why they fail then (could be a QEMU
bug). And the second question of why the test was using -smp 4 on some
machines and not others, which is due to the loop I am trying to remove.
If we can't delete, the loop logic is still a suboptimal way to do
things as qemu reports the max cpus it can take. We could read MAX_SMP
from qemu error output.
Patches welcome, but you'll want to ensure older QEMU also reports the
number of max CPUs. Basically, either we completely drop the loop,
which assumes we're no longer concerned with testing kernels older than
v4.3 and testing shows we always get a working number of CPUs, or we
change the loop to parsing QEMU's output, but that requires testing
all versions of QEMU we care about report the error the same way, or
we leave the loop as is. Alex says the speedup obtained by dropping
the extra QEMU invocations is noticeable, so that's a vote for doing
something, but whatever is chosen will need a commit message identifying
which versions of kernel and/or QEMU are now the oldest ones supported.
Theoretically, I guess we could always have a machine with more CPUs
than QEMU supports. Checking multiple QEMU versions for a specific
message format sounds tedious and flakey. What we really need is a
better way to query QEMU for max cpus and then take min(cpus on machine,
cpus QEMU supports). Any ideas? I sent an email asking the QEMU mailing
list.
