On 7/29/26 12:38, Michael S. Tsirkin wrote:
On Wed, Jul 29, 2026 at 12:25:30PM +0300, Michael Tokarev wrote:
What would break if we switch to the new behavior unconditionally,
regardless of the machine types?
It's guest visible. You will get two reads from same register
suddenly returning different values.
The old value basically makes no sense. We returned a wrong value
here, now we return the correct one. It's like, I don't know, a lot
of fixes in various other areas - when it was a bug in qemu impl. of
some register read which was subsequently corrected - it doesn't need
a compat property to continue returning a wrong/bogus value for old
machines? I think here, there's no real difference - because the
guest can't access the "extra" space anyway without causing some
unexpected results (usually a crash).
I don't understand why do you suggest to implement the same logic for
older/stable qemu versions if it will always evaluate to allocating
the max size for the queue regardless of the requested size.
Thanks,
/mjt
My preference, normally, is to just stick to upstream as much as
possibly. I just do not want slightly different code bases when we can
trivially have one. If nothing else, less of a chance a follow up patch
will cause conflicts, and then it snowballs from there.
Yes, this is exactly my preference as well. It is more, I often
pick up some other changes to stable - changes which aren't fixing
anything, - say, some renames or code shuffling around - just to
make the resulting code closer to the master branch, so that
subsequent changes has much more chances to apply cleanly.
In this case I can drop the addition to hw_compat_11_0[] (because
it doesn't exist in 11.0 and before) and keep everything else.
There will be quite some code which boils down to a single line
(queue_size = QUEUE_SIZE_MAX). Twisted code which does not contain
the main trigger (the property) at all. This change and the
subsequent change which it fixes - in reality, all these ifs
and conditions would be just useless, but will give someone a
puzzle to solve, wtf is going on here?
Ofc it would be ideal - from the back-porting PoV only - to just
drop this all entirely and unconditionally report the correct
queue size to begin with, as per above.
Hwell.. :)
/mjt
