linux-kernel - Re: [PATCH 16/16] x86/entry/64: Move the IST stacks into cpu_entry

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Date:   Tue, 21 Nov 2017 08:38:38 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 16/16] x86/entry/64: Move the IST stacks into
 cpu_entry_area


* Andy Lutomirski <luto@...nel.org> wrote:

>  /* May not be marked __init: used by software suspend */
>  void syscall_init(void)
>  {
> @@ -1627,7 +1637,7 @@ void cpu_init(void)
>  	 * set up and load the per-CPU TSS
>  	 */
>  	if (!oist->ist[0]) {
> -		char *estacks = per_cpu(exception_stacks, cpu);
> +		char *estacks = get_cpu_entry_area(cpu)->exception_stacks;
>  
>  		for (v = 0; v < N_EXCEPTION_STACKS; v++) {
>  			estacks += exception_stack_sizes[v];

This generates a new build warning:

 /home/mingo/tip/arch/x86/kernel/cpu/common.c: In function ‘syscall_init’:
 /home/mingo/tip/arch/x86/kernel/cpu/common.c:1443:6: warning: unused variable ‘cpu’ [-Wunused-variable]
   int cpu = smp_processor_id();

because 'cpu' is now unused in the !CONFIG_IA32_EMULATION part.

The naive fix is something like the patch below, untested.

Thanks,

	Ingo

 arch/x86/kernel/cpu/common.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c7f3a0a19dce..557bad9f4179 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1440,24 +1440,26 @@ void syscall_init(void)
 	extern char _entry_trampoline[];
 	extern char entry_SYSCALL_64_trampoline[];
 
-	int cpu = smp_processor_id();
-
 	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
 	wrmsrl(MSR_LSTAR, (unsigned long)get_cpu_entry_area(smp_processor_id())->entry_trampoline + (entry_SYSCALL_64_trampoline - _entry_trampoline));
 
 #ifdef CONFIG_IA32_EMULATION
-	wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
-	/*
-	 * This only works on Intel CPUs.
-	 * On AMD CPUs these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
-	 * This does not cause SYSENTER to jump to the wrong location, because
-	 * AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit).
-	 */
-	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
-	wrmsrl_safe(MSR_IA32_SYSENTER_ESP,
-		    (unsigned long)&get_cpu_entry_area(cpu)->tss +
-		    offsetofend(struct tss_struct, SYSENTER_stack));
-	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
+	{
+		int cpu = smp_processor_id();
+
+		wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
+		/*
+		 * This only works on Intel CPUs.
+		 * On AMD CPUs these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
+		 * This does not cause SYSENTER to jump to the wrong location, because
+		 * AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit).
+		 */
+		wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
+		wrmsrl_safe(MSR_IA32_SYSENTER_ESP,
+			    (unsigned long)&get_cpu_entry_area(cpu)->tss +
+			    offsetofend(struct tss_struct, SYSENTER_stack));
+		wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
+	}
 #else
 	wrmsrl(MSR_CSTAR, (unsigned long)ignore_sysret);
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);