be retrieved with get_xsave_addr().

Future patches will need to handle the case of writing to an xfeature
that may not be saved. So provide helpers to init an xfeature in an
xsave buffer.

This could of course be done directly by reaching into the xsave buffer,
however this would not be robust against future changes to optimize the
xsave buffer by compacting it. In that case the xsave buffer would need
to be re-arranged as well. So the logic properly belongs encapsulated
in a helper where the logic can be unified.

Tested-by: Pengfei Xu <redacted>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <redacted>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Kees Cook <redacted>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---
v2:
 - New patch
---
 arch/x86/kernel/fpu/xstate.c | 58 +++++++++++++++++++++++++++++-------
 arch/x86/kernel/fpu/xstate.h |  6 ++++
 2 files changed, 53 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 13a80521dd51..3ff80be0a441 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c

@@ -934,6 +934,24 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
 	return (void *)xsave + xfeature_get_offset(xcomp_bv, xfeature_nr);
 }
 
+static int xsave_buffer_access_checks(int xfeature_nr)

+	 * have not enabled.
+	 */
+	if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
+		return 1;
+
+	return 0;
+}
+
 /*
  * Given the xsave area and a state inside, this function returns the
  * address of the state.

@@ -954,17 +972,7 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
  */
 void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
 {
-	/*
-	 * Do we even *have* xsave state?
-	 */
-	if (!boot_cpu_has(X86_FEATURE_XSAVE))
-		return NULL;
-
-	/*
-	 * We should not ever be requesting features that we
-	 * have not enabled.
-	 */
-	if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
+	if (xsave_buffer_access_checks(xfeature_nr))
 		return NULL;
 
 	/*

@@ -984,6 +992,34 @@ void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
 	return __raw_xsave_addr(xsave, xfeature_nr);
 }
 
+/*
+ * Given the xsave area and a state inside, this function
+ * initializes an xfeature in the buffer.