linux-kernel - [PATCH RFC v1 04/11] x86/fpu/xstate: Adjust XSAVE buffer size calculation

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Message-ID: <20250227184502.10288-5-chang.seok.bae@intel.com>
Date: Thu, 27 Feb 2025 10:44:49 -0800
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	chang.seok.bae@...el.com
Subject: [PATCH RFC v1 04/11] x86/fpu/xstate: Adjust XSAVE buffer size calculation

The current xstate size calculation assumes that the highest-numbered
xstate feature has the highest offset in the buffer, determining the size
based on the topmost bit in the feature mask. However, this assumption is
not architecturally guaranteed -- higher-numbered features may have lower
offsets.

With the introduction of the xfeature order table and its helper macro,
xstate components can now be traversed in their positional order. Update
the non-compacted format handling to iterate through the table to
determine the last-positioned feature. Then, set the offset accordingly.

Since size calculation primarily occurs during initialization or in
non-critical paths, looping to find the last feature is not expected to
have a meaningful performance impact.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
---
 arch/x86/kernel/fpu/xstate.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c6f825bb744b..06d2be602ea9 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -576,13 +576,20 @@ static bool __init check_xstate_against_struct(int nr)
 static unsigned int xstate_calculate_size(u64 xfeatures, bool compacted)
 {
 	unsigned int topmost = fls64(xfeatures) -  1;
-	unsigned int offset = xstate_offsets[topmost];
+	unsigned int offset, i;

 	if (topmost <= XFEATURE_SSE)
 		return sizeof(struct xregs_state);

-	if (compacted)
+	if (compacted) {
 		offset = xfeature_get_offset(xfeatures, topmost);
+	} else {
+		/* Walk through the xfeature order to pick the last */
+		for_each_extended_xfeature_in_order(i, xfeatures)
+			topmost = xfeature_noncompact_order[i];
+		offset = xstate_offsets[topmost];
+	}
+
 	return offset + xstate_sizes[topmost];
 }

-- 
2.45.2