KVM: MMU: Avoid calling gfn_to_page() in mmu_set_spte()

Since gfn_to_page() is a sleeping function, and we want to make the core mmu
spinlocked, we need to pass the page from the walker context (which can sleep)
to the shadow context (which cannot).

[marcelo: avoid recursive locking of mmap_sem]

Signed-off-by: Avi Kivity <avi@qumranet.com>

include/asm-x86/kvm_host.h

@@ -224,6 +224,11 @@ struct kvm_vcpu_arch {
 	int   last_pt_write_count;
 	u64  *last_pte_updated;
 
+	struct {
+		gfn_t gfn;          /* presumed gfn during guest pte update */
+		struct page *page;  /* page corresponding to that gfn */
+	} update_pte;
+
 	struct i387_fxsave_struct host_fx_image;
 	struct i387_fxsave_struct guest_fx_image;