KVM: x86: Improve documentation of MSR_KVM_ASYNC_PF_EN

Fix some incorrect statement of MSR_KVM_ASYNC_PF_EN documentation and
state clearly the token in 'struct kvm_vcpu_pv_apf_data' of 'page ready'
event is matchted with the token in CR2 in 'page not present' event.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20231025055914.1201792-3-xiaoyao.li@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

x86/kvm: Use separate percpu variable to track the enabling of asyncpf

Refer to commit fd10cde9294f ("KVM paravirt: Add async PF initialization
to PV guest") and commit 344d9588a9df ("KVM: Add PV MSR to enable
asynchronous page faults delivery"). It turns out that at the time when
asyncpf was introduced, the purpose was defining the shared PV data 'struct
kvm_vcpu_pv_apf_data' with the size of 64 bytes. However, it made a mistake
and defined the size to 68 bytes, which failed to make fit in a cache line
and made the code inconsistent with the documentation.

Below justification quoted from Sean[*]

KVM (the host side) has *never* read kvm_vcpu_pv_apf_data.enabled, and
the documentation clearly states that enabling is based solely on the
bit in the synthetic MSR.

So rather than update the documentation, fix the goof by removing the
enabled filed and use the separate percpu variable instread.
KVM-as-a-host obviously doesn't enforce anything or consume the size,
and changing the header will only affect guests that are rebuilt against
the new header, so there's no chance of ABI breakage between KVM and its
guests. The only possible breakage is if some other hypervisor is
emulating KVM's async #PF (LOL) and relies on the guest to set
kvm_vcpu_pv_apf_data.enabled. But (a) I highly doubt such a hypervisor
exists, (b) that would arguably be a violation of KVM's "spec", and
(c) the worst case scenario is that the guest would simply lose async
#PF functionality.

[*] https://lore.kernel.org/all/ZS7ERnnRqs8Fl0ZF@google.com/T/#u

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20231025055914.1201792-2-xiaoyao.li@intel.com
[sean: use true/false instead of 1/0 for booleans]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Documentation: KVM: add separate directories for architecture-specific documentation

ARM already has an arm/ subdirectory, but s390 and x86 do not even though
they have a relatively large number of files specific to them. Create
new directories in Documentation/virt/kvm for these two architectures
as well.

While at it, group the API documentation and the developer documentation
in the table of contents.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220322110712.222449-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>