[PATCH v6 1/2] x86/mm: add a function to check if a pfn is UC/UC-/WCee

[Haozhong Zhang]

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 determine whether it needs to map a
host pfn to guest with UC memory type.

Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
Reviewed-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>
---
 arch/x86/include/asm/pat.h |  2 ++
 arch/x86/mm/pat.c          | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/x86/include/asm/pat.h b/arch/x86/include/asm/pat.h
index 8a3ee355b422..9a217a18523b 100644
--- a/arch/x86/include/asm/pat.h
+++ b/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_immune_to_uc_mtrr(unsigned long pfn);
+
 #endif /* _ASM_X86_PAT_H */
diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index fe7d57a8fb60..2231a84c3d34 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -677,6 +677,25 @@ static enum page_cache_mode lookup_memtype(u64 paddr)
 	return rettype;
 }
 
+/**
+ * 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_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_immune_to_uc_mtrr);
+
 /**
  * io_reserve_memtype - Request a memory type mapping for a region of memory
  * @start: start (physical address) of the region
-- 
2.14.1