Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 03, 2020, David Woodhouse wrote:
> On Wed, 2020-12-02 at 12:32 -0800, Ankur Arora wrote:
> > > On IRC, Paolo told me that permanent pinning causes problems for memory
> > > hotplug, and pointed me at the trick we do with an MMU notifier and
> > > kvm_vcpu_reload_apic_access_page().
> > 
> > Okay that answers my question. Thanks for clearing that up.
> > 
> > Not sure of a good place to document this but it would be good to
> > have this written down somewhere. Maybe kvm_map_gfn()?
> 
> Trying not to get too distracted by polishing this part, so I can
> continue with making more things actually work. But I took a quick look
> at the reload_apic_access_page() thing.
> 
> AFAICT it works because the access is only from *within* the vCPU, in
> guest mode.
> 
> So all the notifier has to do is kick all CPUs, which happens when it
> calls kvm_make_all_cpus_request(). Thus we are guaranteed that all CPUs
> are *out* of guest mode by the time...
> 
>     ...er... maybe not by the time the notifier returns, because all 
>     we've done is *send* the IPI and we don't know the other CPUs have 
>     actually stopped running the guest yet? 
> 
>     Maybe there's some explanation of why the actual TLB shootdown 
>     truly *will* occur before the page goes away, and some ordering 
>     rules which mean our reschedule IPI will happen first? Something 
>     like that ideally would have been in a comment in in MMU notifier.

KVM_REQ_APIC_PAGE_RELOAD is tagged with KVM_REQUEST_WAIT, which means that
kvm_kick_many_cpus() and thus smp_call_function_many() will have @wait=true,
i.e. the sender will wait for the SMP function call to finish on the target CPUs.