On Fri, Apr 12, 2013 at 05:16:20PM +0200, Igor Mammedov wrote: > On Fri, 12 Apr 2013 10:35:53 -0300 > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > On Fri, Apr 12, 2013 at 12:53:51PM +0200, Igor Mammedov wrote: > > > On Thu, 11 Apr 2013 15:59:40 -0300 > > > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > > > > > On Thu, Apr 11, 2013 at 04:51:46PM +0200, Igor Mammedov wrote: > > > > > ... so that on reboot BIOS could read current available CPU count > > > > > > > > > > Signed-off-by: Igor Mammedov <imamm...@redhat.com> > > > > > v2: > > > > > * > > > > > s/qemu_register_cpu_add_notifier()/qemu_register_cpu_added_notifier()/ > > > > > --- > > > > > hw/timer/mc146818rtc.c | 12 ++++++++++++ > > > > > 1 file changed, 12 insertions(+) > > > > > > > > Initialization of the cmos fields (including 0x5F) is done on > > > > pc.c:pc_cmos_init(). What about making the field increment inside pc.c > > > > as well? > > > I looked at possibility but discarded it because to increment it there > > > initial > > > value should be -1 (field is zero based) which is not obvious, plug ugly > > > casting to singed variable. > > > Result looked ugly. > > > > I was thinking about simply adding exactly the same code with exactly > > the same logic, but inside pc.c instead of of mc146818rtc.c. Instead of > > registering the notifier inside rtc_initfn(), register exactly the same > > notifier with exactly the same code, but inside pc_cmos_init() (that's > > where 0x5f is initialized). > > > > It would even be safer and easier review and ensure correctness: with > > this patch, the notifier is registered very early, even before > > pc_cmos_init() initializes 0x5f to smp_cpus. CPU hotplug events are > > unlikely to be emitted before pc_cmos_init() is called, but still: why > it isn't be called, hot-add is available only after machine initialized. > > > make the initialization ordering so subtle if we don't have to? > Currently cmos init doesn't look like proper QOM object and has 3 stage > initialization: realize(), then pc_cmos_init() the last pc_cmos_init_late(). > The last 2 calls are made after realize(), setting various properties. Which > looks wrong from QOM perspective, so I'm against of stuffing more internal > stuff in arbitrary places. We should do opposite instead.
True, but as we already have this weird 3-stage initialization process and we won't fix it really soon, I would really prefer to keep parts of the code that are closely related and depend on each other in the same part of the code. > > If you look at mc146818rtc.c or hw/acpi/piix4.c, all notifiers are private to > object and registered at realize() time. It looks like initialization order > of mc146818rtc should be fixed, instead of adapting new code to it. > > So since this patch doesn't break or violate anything in current code, I'd > like to leave it as it is. If you insist into making the mc146818rtc device take care of maintaining the 0x5f value by itself, why not doing: s->cmos_data[0x5f] = smp_cpus - 1; inside rtc_initfn() instead of pc_cmos_init() as well? This would be one additional step towards making pc_cmos_init() be replaced by QOM-based code (if that's what you want to do in the long term). > > > > > > > > > What happens if a CPU is hotplugged after the machine has started but > > > > before the guest OS has booted? Are we supposed to make sure the BIOS do > > > > the right thing if a CPU is hotplugged before the OS has booted, or this > > > > simply won't be supported? > > > BIOS uses this value to set in ACPI tables what CPUs are present. > > > 1. if hot-plug happens before BIOS reads it then OS will see all CPUs > > > and SCI it receives will be nop. > > > 2. if hot-plug happens after BIOS reads it, OS will handle SCI as usual > > > and hotplug CPU instead of initializing it smp_boot() time. > > > BIOS itself has nothing to do with hot-plug, it's OSPM job. > > > > Makes sense, thanks. > > > > What happens if the CPU is hotplugged after the BIOS builds the ACPI > > tables, but long before the OS starts handling ACPI events? Is the OS > > guaranteed to run the CPU hotplug ACPI method (that's \_GPE._E02 in the > > DSDT, right?), even if that happens? > Theoretically interrupt should not disappear on it's own and OS should pick it > up. But to say it for sure I need to test this case. If SCI will be lost for > some reason, then OS won't notice new CPU until another CPU hot-plug event > happens. Makes sense, thanks. (Anyway, if an interrupt gets lost, it's probably a BIOS or OS bug). > [...] -- Eduardo