x86/mm: add a function to check if a pfn is UC/UC-/WC

Check whether the PAT memory type of a pfn cannot be overridden by
MTRR UC memory type, i.e. the PAT memory type is UC, UC- or WC. This
function will be used by KVM to distinguish MMIO pfns and give them
UC memory type in the EPT page tables (on Intel processors, EPT
memory types work like MTRRs).

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Xiao Guangrong <xiaoguangrong@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/include/asm/pat.h

@@ -22,4 +22,6 @@ int io_reserve_memtype(resource_size_t start, resource_size_t end,
 
 void io_free_memtype(resource_size_t start, resource_size_t end);
 
+bool pat_pfn_immune_to_uc_mtrr(unsigned long pfn);
+
 #endif /* _ASM_X86_PAT_H */

arch/x86/mm/pat.c

@@ -677,6 +677,25 @@ static enum page_cache_mode lookup_memtype(u64 paddr)
 	return rettype;
 }
 
+/**
+ * pat_pfn_immune_to_uc_mtrr - Check whether the PAT memory type
+ * of @pfn cannot be overridden by UC MTRR memory type.
+ *
+ * Only to be called when PAT is enabled.
+ *
+ * Returns true, if the PAT memory type of @pfn is UC, UC-, or WC.
+ * Returns false in other cases.
+ */
+bool pat_pfn_immune_to_uc_mtrr(unsigned long pfn)
+{
+	enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
+
+	return cm == _PAGE_CACHE_MODE_UC ||
+	       cm == _PAGE_CACHE_MODE_UC_MINUS ||
+	       cm == _PAGE_CACHE_MODE_WC;
+}
+EXPORT_SYMBOL_GPL(pat_pfn_immune_to_uc_mtrr);
+
 /**
  * io_reserve_memtype - Request a memory type mapping for a region of memory
  * @start: start (physical address) of the region