linux-kernel - Re: [PATCH v11 35/37] x86/syscall: Split IDT syscall setup code into idt_syscall

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Date:   Mon, 25 Sep 2023 09:07:47 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Xin Li <xin3.li@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
CC:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, luto@...nel.org,
        pbonzini@...hat.com, seanjc@...gle.com, peterz@...radead.org,
        jgross@...e.com, ravi.v.shankar@...el.com, mhiramat@...nel.org,
        andrew.cooper3@...rix.com, jiangshanlai@...il.com,
        nik.borisov@...e.com
Subject: Re: [PATCH v11 35/37] x86/syscall: Split IDT syscall setup code into idt_syscall_init()

On September 23, 2023 2:42:10 AM PDT, Xin Li <xin3.li@...el.com> wrote:
>Because FRED uses the ring 3 FRED entrypoint for SYSCALL and SYSENTER and
>ERETU is the only legit instruction to return to ring 3, there is NO need
>to setup SYSCALL and SYSENTER MSRs for FRED, except the IA32_STAR MSR.
>
>Split IDT syscall setup code into idt_syscall_init() to make it easy to
>skip syscall setup code when FRED is enabled.
>
>Suggested-by: Thomas Gleixner <tglx@...utronix.de>
>Tested-by: Shan Kang <shan.kang@...el.com>
>Signed-off-by: Xin Li <xin3.li@...el.com>
>---
> arch/x86/kernel/cpu/common.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
>diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>index 20bbedbf6dcb..2ee4e7b597a3 100644
>--- a/arch/x86/kernel/cpu/common.c
>+++ b/arch/x86/kernel/cpu/common.c
>@@ -2071,10 +2071,8 @@ static void wrmsrl_cstar(unsigned long val)
> 		wrmsrl(MSR_CSTAR, val);
> }
> 
>-/* May not be marked __init: used by software suspend */
>-void syscall_init(void)
>+static inline void idt_syscall_init(void)
> {
>-	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
> 	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
> 
> 	if (ia32_enabled()) {
>@@ -2108,6 +2106,15 @@ void syscall_init(void)
> 	       X86_EFLAGS_AC|X86_EFLAGS_ID);
> }
> 
>+/* May not be marked __init: used by software suspend */
>+void syscall_init(void)
>+{
>+	/* The default user and kernel segments */
>+	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
>+
>+	idt_syscall_init();
>+}
>+
> #else	/* CONFIG_X86_64 */
> 
> #ifdef CONFIG_STACKPROTECTOR

Am I missing something, or is this patch a noop?