Re: [Qemu-devel] [PATCH v2 1/6] Use IO port for qemu<->guest BIOS communication.

[Avi Kivity <avi@qumranet.com>]

Blue Swirl wrote:
>
> One problem with this IO method is that it is not SMP safe, because of
> the cur_entry selection and reading will not be atomic. Think of boot
> CPU waking up all other CPUs with broadcast interrupt and then these
> other CPUs all want to know the boot parameters at the same time. The
> same could happen with warm reset. Directly mapped ROM doesn't have
> this problem, but multiplexed ROM (or Sparc64 nvram used now) is also
> unsafe.
>
> I could think of a couple of solutions:
> - different IO port for each CPU (not possible on PC)
> - per-CPU cur_entry, the device "knows" the CPU index (not very beautiful)
> - lock mechanism: before access, the CPU must win lock and only then
> it can access the data. But IO ports don't support atomic
> instructions? How about some kind of MAC-like protocol? It gets pretty
> complex.
>
> Are there other solutions?
>
>   

Put the lock in memory.  Or even simpler, have the boot cpu extract the
parameters and place them in memory, and then wake up other cpus.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.