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

[Paolo Bonzini]

On 25/06/2015 19:08, Andreas Färber wrote:
> > I said "I think this patch is incorrect, because you do not install a
> > separate address space for each CPU.  Also, the CPU address space is
> > only used with TCG so it should be guarded by "if (tcg_enabled())"."
> > 
> > By the way, now TCG _is_ installing a separate address space per CPU
> > already, so the patch can simply guard the code with "if (tcg_enabled())".
> 
> Is the APIC MemoryRegion not used by KVM?

It's used if the userspace LAPIC is.  It's not used together with the
kernel LAPIC (accesses really are trapped by the kernel).

> Otherwise if we still need the
> ugly code path for KVM, that's not much of an improvement here.
> 
> 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.

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

Paolo