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

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Message-ID: <216ec775-2f75-cb70-a2ef-0884603d8c18@virtuozzo.com>
Date:   Tue, 21 Nov 2017 17:45:06 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Ingo Molnar <mingo@...nel.org>, 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



On 11/21/2017 10:38 AM, Ingo Molnar wrote:
> 
> * 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));

             It would be better to use 'cpu' variable here  ^^^^^^^^^^^^^^^^^
>  
>  #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);
>