Re: [Qemu-devel] [PATCH 06/11] target-i386: add socket/core/thread properties to X86CPU

[Eduardo Habkost]

On Thu, Jun 23, 2016 at 10:46:36PM +0200, Igor Mammedov wrote:
> On Thu, 23 Jun 2016 17:18:46 -0300
> Eduardo Habkost <address@hidden> wrote:
[...]
> > > 
> > > > 
> > > > I suggest validating the properties, and setting them in case
> > > > they are not set:
> > > > 
> > > >     x86_topo_ids_from_apicid(cpu->apic_id, smp_cores, smp_threads,
> > > > &topo);
> > > > 
> > > >     if (cpu->socket != -1 && cpu->socket != topo.socket_id) {
> > > >         error_setg(errp, "CPU socket ID mismatch: ...");
> > > >         return;
> > > >     }
> > > >     cpu->socket = topo.socket_id;
> > > > 
> > > >     if (cpu->core != -1 && cpu->core != topo.core_id) {
> > > >         error_setg(errp, "CPU core ID mismatch: ...");
> > > >         return;
> > > >     }
> > > >     cpu->core = topo.core_id;
> > > > 
> > > >     if (cpu->thread != -1 && cpu->thread != topo.smt_id) {
> > > >         error_setg(errp, "CPU thread ID mismatch: ...");
> > > >         return;
> > > >     }
> > > >     cpu->thread = topo.smt_id;
> > > > 
> > > > We could do that inside x86_cpu_realizefn(), so that
> > > > socket/core/thread would be always set in all CPUs.
> > > all CPUs pass through pc_cpu_pre_plug() before cpu.relizefn()
> > > so I'd rather do it here at board level responsible for
> > > setting apic_id or socket/core/thread info is not set.
> > 
> > Then *-user will be inconsistent, and will always have invalid
> > values in socket/core/thread. If one day we add any logic using
> > the socket/core/thread properties in cpu.c, it will break on
> > *-user.
> > 
> > All those properties are X86CPU properties, meaning they are
> > input to the X86CPU code. I don't see why we should move logic
> > related to them outside cpu.c unless really necessary.
> > 
> > (The apic-id calculation at patch 07/11, for example, is more
> > difficult to move to cpu.c, because the PC code needs the APIC ID
> > before calling realize. We could move it to the apic-id getter,
> > but I dislike having magic getter/setters and like that you made
> > it a static property.)
> set of socket/core/thread is a synonym for apic_id and it's board that
> manages and knows valid values for them.

The CPU already knows exactly what the bits inside APIC ID mean,
because it has to report topology information through CPUID.

> Putting above snippet into
> cpu.relizefn() would make CPU access globals smp_cores, smp_threads
> which are essentially machine_state and Drew working on moving them
> there and eliminating globals. So suddenly CPU would need to poke into
> machine object, and we return to the same state wrt *-user only with
> hack in cpu.c.

After Drew's code is included, we can simply use
CPUState::nr_cores and CPUState::nr_threads.

> 
> I'd worry about -smp and *-user when it comes into that target as it
> will probably need apic_id and maybe socket/core/thread as well.

The point is to not even have to worry about *-user later, by
keeping both softmmu and *-user consistent. People reviewing
patches a few years from now probably wouldn't even notice that
code using the socket/core/thread fields will break in *-user.

I wouldn't mind about having the code in pc.c at all, if it
didn't make *-user inconsistent, or if the CPU object didn't had
all the required information yet. But I don't think it is
reasonable to intentionally leave X86CPU fields inconsistent in
*-user if we can easily fix it by moving initialization to
realizefn.

But if you are really strongly against that, I can propose that
as a follow-up later (after Drew's series is included).

-- 
Eduardo