[RFC PATCH 03/12] x86/fpu/xstate: Add xsaves_nmi

[kan.liang@linux.intel.com]

From: Kan Liang <kan.liang@linux.intel.com>

Linux perf_event subsystem needs to retrieve the current vector
registers in an overflow. The overflow is handled by NMI.

Add an interface to retrieve the actual register contents when the NMI
hit. It's the invoker's responsibility to make sure the contents are
properly filtered before exposing them to the end user.

The mask may be changed according to the end user's request. The XSAVES
with the modified optimizations are chosen.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
 arch/x86/include/asm/fpu/xstate.h |  1 +
 arch/x86/kernel/fpu/xstate.c      | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index b308a76afbb7..87c170d61138 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -106,6 +106,7 @@ extern void __init update_regset_xstate_info(unsigned int size,
 int xfeature_size(int xfeature_nr);
 
 void xsaves(struct xregs_state *xsave, u64 mask);
+void xsaves_nmi(struct xregs_state *xsave, u64 mask);
 void xrstors(struct xregs_state *xsave, u64 mask);
 
 int xfd_enable_feature(u64 xfd_err);
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 9aa9ac8399ae..5b0bae135aff 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1424,6 +1424,28 @@ void xsaves(struct xregs_state *xstate, u64 mask)
 	WARN_ON_ONCE(err);
 }
 
+/**
+ * xsaves_nmi - Save selected components to a kernel xstate buffer in NMI
+ * @xstate:	Pointer to the buffer
+ * @mask:	Feature mask to select the components to save
+ *
+ * The @xstate buffer must be 64 byte aligned and correctly initialized as
+ * XSAVES does not write the full xstate header.
+ *
+ * This function can only be invoked in an NMI. It returns the *ACTUAL*
+ * register contents when the NMI hit.
+ */
+void xsaves_nmi(struct xregs_state *xstate, u64 mask)
+{
+	int err;
+
+	if (!in_nmi())
+		return;
+
+	XSTATE_OP(XSAVES, xstate, (u32)mask, (u32)(mask >> 32), err);
+	WARN_ON_ONCE(err);
+}
+
 /**
  * xrstors - Restore selected components from a kernel xstate buffer
  * @xstate:	Pointer to the buffer
-- 
2.38.1