Revert "KVM: MMU: document fast invalidate all pages"

Remove x86 KVM's fast invalidate mechanism, i.e. revert all patches
from the original series[1].

Though not explicitly stated, for all intents and purposes the fast
invalidate mechanism was added to speed up the scenario where removing
a memslot, e.g. as part of accessing reading PCI ROM, caused KVM to
flush all shadow entries[1].  Now that the memslot case flushes only
shadow entries belonging to the memslot, i.e. doesn't use the fast
invalidate mechanism, the only remaining usage of the mechanism are
when the VM is being destroyed and when the MMIO generation rolls
over.

When a VM is being destroyed, either there are no active vcpus, i.e.
there's no lock contention, or the VM has ungracefully terminated, in
which case we want to reclaim its pages as quickly as possible, i.e.
not release the MMU lock if there are still CPUs executing in the VM.

The MMIO generation scenario is almost literally a one-in-a-million
occurrence, i.e. is not a performance sensitive scenario.

Given that lock-breaking is not desirable (VM teardown) or irrelevant
(MMIO generation overflow), remove the fast invalidate mechanism to
simplify the code (a small amount) and to discourage future code from
zapping all pages as using such a big hammer should be a last restort.

This reverts commit f6f8adee.

[1] https://lkml.kernel.org/r/1369960590-14138-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com

Cc: Xiao Guangrong <guangrong.xiao@gmail.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -224,10 +224,6 @@ Shadow pages contain the following information:
     A bitmap indicating which sptes in spt point (directly or indirectly) at
     pages that may be unsynchronized.  Used to quickly locate all unsychronized
     pages reachable from a given page.
-  mmu_valid_gen:
-    Generation number of the page.  It is compared with kvm->arch.mmu_valid_gen
-    during hash table lookup, and used to skip invalidated shadow pages (see
-    "Zapping all pages" below.)
   clear_spte_count:
     Only present on 32-bit hosts, where a 64-bit spte cannot be written
     atomically.  The reader uses this while running out of the MMU lock
@@ -402,27 +398,6 @@ causes its disallow_lpage to be incremented, thus preventing instantiation of
 a large spte.  The frames at the end of an unaligned memory slot have
 artificially inflated ->disallow_lpages so they can never be instantiated.
 
-Zapping all pages (page generation count)
-=========================================
-
-For the large memory guests, walking and zapping all pages is really slow
-(because there are a lot of pages), and also blocks memory accesses of
-all VCPUs because it needs to hold the MMU lock.
-
-To make it be more scalable, kvm maintains a global generation number
-which is stored in kvm->arch.mmu_valid_gen.  Every shadow page stores
-the current global generation-number into sp->mmu_valid_gen when it
-is created.  Pages with a mismatching generation number are "obsolete".
-
-When KVM need zap all shadow pages sptes, it just simply increases the global
-generation-number then reload root shadow pages on all vcpus.  As the VCPUs
-create new shadow page tables, the old pages are not used because of the
-mismatching generation number.
-
-KVM then walks through all pages and zaps obsolete pages.  While the zap
-operation needs to take the MMU lock, the lock can be released periodically
-so that the VCPUs can make progress.
-
 Fast invalidation of MMIO sptes
 ===============================
 
@@ -435,8 +410,7 @@ shadow pages, and is made more scalable with a similar technique.
 MMIO sptes have a few spare bits, which are used to store a
 generation number.  The global generation number is stored in
 kvm_memslots(kvm)->generation, and increased whenever guest memory info
-changes.  This generation number is distinct from the one described in
-the previous section.
+changes.
 
 When KVM finds an MMIO spte, it checks the generation number of the spte.
 If the generation number of the spte does not equal the global generation

arch/x86/include/asm/kvm_host.h

@@ -333,10 +333,7 @@ struct kvm_mmu_page {
 	int root_count;          /* Currently serving as active root */
 	unsigned int unsync_children;
 	struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
-
-	/* The page is obsolete if mmu_valid_gen != kvm->arch.mmu_valid_gen.  */
 	unsigned long mmu_valid_gen;
-
 	DECLARE_BITMAP(unsync_child_bitmap, 512);
 
 #ifdef CONFIG_X86_32