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 11:55:50 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: RE: [PATCH v11 35/37] x86/syscall: Split IDT syscall setup code into idt_syscall_init()

On September 25, 2023 10:56:44 AM PDT, "Li, Xin3" <xin3.li@...el.com> wrote:
>> >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?
>
>Yes, this is a noop, just a cleanup patch w/o functionality change.
>
>

It just seems to be completely redundant. We can just drop it, no? If we aren't going to explicitly clobber the registers there is no harm in setting them up for IDT unconditionally.