linux-kernel - Re: [PATCH v2] x86/fred: Fix 64bit identifier in fred

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Message-ID: <489572CC-89F3-4546-AE42-736B6D6F1657@zytor.com>
Date: Tue, 02 Sep 2025 17:01:30 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>
CC: Xin Li <xin@...or.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Subject: Re: [PATCH v2] x86/fred: Fix 64bit identifier in fred_ss

On September 2, 2025 4:01:17 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>FRED can only be enabled in Long Mode.  This is the 64bit mode (as opposed to
>compatibility mode) identifier, rather than being something hard-wired at 1.
>
>No functional change.
>
>Reviewed-by: Xin Li (Intel) <xin@...or.com>
>Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
>Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
>---
>CC: Xin Li <xin@...or.com>
>CC: "H. Peter Anvin" <hpa@...or.com>
>CC: Andy Lutomirski <luto@...nel.org>
>CC: Thomas Gleixner <tglx@...utronix.de>
>CC: Ingo Molnar <mingo@...hat.com>
>CC: Borislav Petkov <bp@...en8.de>
>CC: Dave Hansen <dave.hansen@...ux.intel.com>
>CC: x86@...nel.org
>CC: linux-kernel@...r.kernel.org
>
>v2:
> * State no functional change.
>
>IIRC, this was wrong in an older version of the FRED spec.  I made the same
>mistake in Xen, and I didn't copy Linux's structure.
>
>I don't thinks this warrants backporting, but the fixes tag is:
>
>Fixes: 3c77bf02d0c0 ("x86/ptrace: Add FRED additional information to the pt_regs structure")
>---
> arch/x86/entry/entry_fred.c   | 4 ++--
> arch/x86/include/asm/fred.h   | 2 +-
> arch/x86/include/asm/ptrace.h | 4 ++--
> 3 files changed, 5 insertions(+), 5 deletions(-)
>
>diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
>index f004a4dc74c2..94e626cc6a07 100644
>--- a/arch/x86/entry/entry_fred.c
>+++ b/arch/x86/entry/entry_fred.c
>@@ -78,13 +78,13 @@ static noinstr void fred_intx(struct pt_regs *regs)
> static __always_inline void fred_other(struct pt_regs *regs)
> {
> 	/* The compiler can fold these conditions into a single test */
>-	if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.lm)) {
>+	if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.l)) {
> 		regs->orig_ax = regs->ax;
> 		regs->ax = -ENOSYS;
> 		do_syscall_64(regs, regs->orig_ax);
> 		return;
> 	} else if (ia32_enabled() &&
>-		   likely(regs->fred_ss.vector == FRED_SYSENTER && !regs->fred_ss.lm)) {
>+		   likely(regs->fred_ss.vector == FRED_SYSENTER && !regs->fred_ss.l)) {
> 		regs->orig_ax = regs->ax;
> 		regs->ax = -ENOSYS;
> 		do_fast_syscall_32(regs);
>diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
>index 12b34d5b2953..2bb65677c079 100644
>--- a/arch/x86/include/asm/fred.h
>+++ b/arch/x86/include/asm/fred.h
>@@ -79,7 +79,7 @@ static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int
> 		.type   = type,
> 		.vector = vector,
> 		.nmi    = type == EVENT_TYPE_NMI,
>-		.lm     = 1,
>+		.l      = 1,
> 	};
> 
> 	asm_fred_entry_from_kvm(ss);
>diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
>index 50f75467f73d..37370c3b1ffd 100644
>--- a/arch/x86/include/asm/ptrace.h
>+++ b/arch/x86/include/asm/ptrace.h
>@@ -84,8 +84,8 @@ struct fred_ss {
> 			:  4,
> 		/* Event was incident to enclave execution */
> 		enclave	:  1,
>-		/* CPU was in long mode */
>-		lm	:  1,
>+		/* CPU was in 64-bit mode */
>+		l	:  1,
> 		/*
> 		 * Nested exception during FRED delivery, not set
> 		 * for #DF.
>
>base-commit: e6b9dce0aeeb91dfc0974ab87f02454e24566182

Acked-by: H. Peter Anvin (Intel) <hpa@...or.com>