Re: [RFC PATCH v4 16/28] x86: Add support for changing memory encryption attribute

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On 2/22/2017 12:52 PM, Borislav Petkov wrote:
On Thu, Feb 16, 2017 at 09:45:35AM -0600, Tom Lendacky wrote:
Add support for changing the memory encryption attribute for one or more
memory pages.

"This will be useful when we, ...., for example."

Yup, will expand on the "why".


Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 arch/x86/include/asm/cacheflush.h |    3 ++
 arch/x86/mm/pageattr.c            |   66 +++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
index 872877d..33ae60a 100644
--- a/arch/x86/include/asm/cacheflush.h
+++ b/arch/x86/include/asm/cacheflush.h
@@ -12,6 +12,7 @@
  * Executability : eXeutable, NoteXecutable
  * Read/Write    : ReadOnly, ReadWrite
  * Presence      : NotPresent
+ * Encryption    : Encrypted, Decrypted
  *
  * Within a category, the attributes are mutually exclusive.
  *
@@ -47,6 +48,8 @@
 int set_memory_rw(unsigned long addr, int numpages);
 int set_memory_np(unsigned long addr, int numpages);
 int set_memory_4k(unsigned long addr, int numpages);
+int set_memory_encrypted(unsigned long addr, int numpages);
+int set_memory_decrypted(unsigned long addr, int numpages);

 int set_memory_array_uc(unsigned long *addr, int addrinarray);
 int set_memory_array_wc(unsigned long *addr, int addrinarray);
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 91c5c63..9710f5c 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1742,6 +1742,72 @@ int set_memory_4k(unsigned long addr, int numpages)
 					__pgprot(0), 1, 0, NULL);
 }

+static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
+{
+	struct cpa_data cpa;
+	unsigned long start;
+	int ret;
+
+	/* Nothing to do if the _PAGE_ENC attribute is zero */
+	if (_PAGE_ENC == 0)

Why not:

	if (!sme_active())

?

Yup, it would be more clear.


+		return 0;
+
+	/* Save original start address since it will be modified */

That's obvious - it is a small-enough function to fit on the screen. No
need for the comment.

Ok.


+	start = addr;
+
+	memset(&cpa, 0, sizeof(cpa));
+	cpa.vaddr = &addr;
+	cpa.numpages = numpages;
+	cpa.mask_set = enc ? __pgprot(_PAGE_ENC) : __pgprot(0);
+	cpa.mask_clr = enc ? __pgprot(0) : __pgprot(_PAGE_ENC);
+	cpa.pgd = init_mm.pgd;
+
+	/* Should not be working on unaligned addresses */
+	if (WARN_ONCE(*cpa.vaddr & ~PAGE_MASK,
+		      "misaligned address: %#lx\n", *cpa.vaddr))

Use addr here so that you don't have to deref. gcc is probably smart
enough but the code should look more readable this way too.


Ok.

+		*cpa.vaddr &= PAGE_MASK;

I know, you must use cpa.vaddr here but if you move that alignment check
over the cpa assignment, you can use addr solely.

Ok.


+
+	/* Must avoid aliasing mappings in the highmem code */
+	kmap_flush_unused();
+	vm_unmap_aliases();
+
+	/*
+	 * Before changing the encryption attribute, we need to flush caches.
+	 */
+	if (static_cpu_has(X86_FEATURE_CLFLUSH))
+		cpa_flush_range(start, numpages, 1);
+	else
+		cpa_flush_all(1);

I guess we don't really need the distinction since a SME CPU most
definitely implies CLFLUSH support but ok, let's be careful.

+
+	ret = __change_page_attr_set_clr(&cpa, 1);
+
+	/*
+	 * After changing the encryption attribute, we need to flush TLBs
+	 * again in case any speculative TLB caching occurred (but no need
+	 * to flush caches again).  We could just use cpa_flush_all(), but
+	 * in case TLB flushing gets optimized in the cpa_flush_range()
+	 * path use the same logic as above.
+	 */
+	if (static_cpu_has(X86_FEATURE_CLFLUSH))
+		cpa_flush_range(start, numpages, 0);
+	else
+		cpa_flush_all(0);
+
+	return ret;
+}


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