KVM: x86/mmu: Detect write #PF to shadow pages during FNAME(fetch) walk

Move the detection of write #PF to shadow pages, i.e. a fault on a write
to a page table that is being shadowed by KVM that is used to translate
the write itself, from FNAME(is_self_change_mapping) to FNAME(fetch).
There is no need to detect the self-referential write before
kvm_faultin_pfn() as KVM does not consume EMULTYPE_WRITE_PF_TO_SP for
accesses that resolve to "error or no-slot" pfns, i.e. KVM doesn't allow
retrying MMIO accesses or writes to read-only memslots.

Detecting the EMULTYPE_WRITE_PF_TO_SP scenario in FNAME(fetch) will allow
dropping FNAME(is_self_change_mapping) entirely, as the hugepage
interaction can be deferred to kvm_mmu_hugepage_adjust().

Cc: Huang Hang <hhuang@linux.alibaba.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20221213125538.81209-1-jiangshanlai@gmail.com
[sean: split to separate patch, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230202182817.407394-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Lai Jiangshan

arch/x86/kvm/mmu/paging_tmpl.h

@@ -685,6 +685,9 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 
 		if (sp != ERR_PTR(-EEXIST))
 			link_shadow_page(vcpu, it.sptep, sp);
+
+		if (fault->write && table_gfn == fault->gfn)
+			fault->write_fault_to_shadow_pgtable = true;
 	}
 
 	kvm_mmu_hugepage_adjust(vcpu, fault);
@@ -741,17 +744,13 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
  * created when kvm establishes shadow page table that stop kvm using large
  * page size. Do it early can avoid unnecessary #PF and emulation.
  *
- * @write_fault_to_shadow_pgtable will return true if the fault gfn is
- * currently used as its page table.
- *
  * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
  * since the PDPT is always shadowed, that means, we can not use large page
  * size to map the gfn which is used as PDPT.
  */
 static bool
 FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
-			      struct guest_walker *walker, bool user_fault,
-			      bool *write_fault_to_shadow_pgtable)
+			      struct guest_walker *walker, bool user_fault)
 {
 	int level;
 	gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
@@ -765,7 +764,6 @@ FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
 		gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
 
 		self_changed |= !(gfn & mask);
-		*write_fault_to_shadow_pgtable |= !gfn;
 	}
 
 	return self_changed;
@@ -826,7 +824,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 		return r;
 
 	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
-	      &walker, fault->user, &fault->write_fault_to_shadow_pgtable);
+	      &walker, fault->user);
 
 	if (is_self_change_mapping)
 		fault->max_level = PG_LEVEL_4K;