Re: [Qemu-devel] [RESEND PATCH v8 1/4] apic: map APIC's MMIO region at each CPU's address space

[Peter Maydell]

On 25 June 2015 at 18:27, Paolo Bonzini <address@hidden> wrote:
> On 25/06/2015 19:08, Andreas Färber wrote:
>> And is installing a separate address space per CPU for KVM difficult due
>> to kernel limitations, or is this just a few lines of QEMU code that Zhu
>> or someone would need to write? :)
>
> It's basically impossible.  Even though support for multiple address
> spaces is going to be in Linux 4.2, there are going to be just two: SMM
> and not SMM.  You don't really want to do O(#cpus) stuff in KVM, where
> the number of CPUs can be 200 or more.

Can you explain what the issue is here? Shouldn't it just be a matter
of kvm_cpu_exec() doing a dispatch to cpu->as rather than calling
address_space_rw() ?  (Making it do that was one of the things on my
todo list for ARM at some point.)

I'm happy to assume that RAM is shared by all CPUs I guess.

> TCG is okay because the #cpus is not really going to be more than 4-ish.

Well, it might be more than that in future...

-- PMM