linux-kernel - [PATCH] x86/fpu: Correct AVX512 state tracking

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Date:   Tue, 16 Nov 2021 17:21:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Noah Goldstein <goldstein.w.n@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        hpa@...or.com, luto@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] x86/fpu: Correct AVX512 state tracking

Does that still work with your test case? (Ontop of -rc1)

---
From: Noah Goldstein <goldstein.w.n@...il.com>
Date: Tue, 16 Nov 2021 17:14:21 +0100
Subject: [PATCH] x86/fpu: Correct AVX512 state tracking

Add a separate, local mask for tracking AVX512 usage which does not
include the opmask xfeature set. Opmask registers usage does not cause
frequency throttling so it is a completely unnecessary false positive.

While at it, carve it out into a separate function to keep that
abomination extracted out.

 [ bp: Rediff and cleanup ontop of 5.16-rc1. ]

Signed-off-by: Noah Goldstein <goldstein.w.n@...il.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lore.kernel.org/r/20210920053951.4093668-1-goldstein.w.n@gmail.com
---
 arch/x86/kernel/fpu/core.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 8ea306b1bf8e..dd3777ac0443 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -98,6 +98,19 @@ bool irq_fpu_usable(void)
 }
 EXPORT_SYMBOL(irq_fpu_usable);
 
+/*
+ * Track AVX512 state use because it is known to slow the max clock
+ * speed of the core.
+ */
+static void update_avx_timestamp(struct fpu *fpu)
+{
+
+#define AVX512_TRACKING_MASK	(XFEATURE_MASK_ZMM_Hi256 | XFEATURE_MASK_Hi16_ZMM)
+
+	if (fpu->fpstate->regs.xsave.header.xfeatures & AVX512_TRACKING_MASK)
+		fpu->avx512_timestamp = jiffies;
+}
+
 /*
  * Save the FPU register state in fpu->fpstate->regs. The register state is
  * preserved.
@@ -116,13 +129,7 @@ void save_fpregs_to_fpstate(struct fpu *fpu)
 {
 	if (likely(use_xsave())) {
 		os_xsave(fpu->fpstate);
-
-		/*
-		 * AVX512 state is tracked here because its use is
-		 * known to slow the max clock speed of the core.
-		 */
-		if (fpu->fpstate->regs.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
-			fpu->avx512_timestamp = jiffies;
+		update_avx_timestamp(fpu);
 		return;
 	}
 
-- 
2.29.2


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette