linux-kernel - Re: [PATCH v2 2/2] x86: disable IRQs before changing CR4

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Date:   Sat, 25 Nov 2017 09:20:23 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-edac@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Tony Luck <tony.luck@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 2/2] x86: disable IRQs before changing CR4

Thomas Gleixner <tglx@...utronix.de> wrote:

> On Fri, 24 Nov 2017, Nadav Amit wrote:
>> /* Set in this cpu's CR4. */
>> -static inline void cr4_set_bits(unsigned long mask)
>> +static inline void cr4_set_bits_irqs_off(unsigned long mask)
> 
> This change is kinda weird. I'd expect that there is a corresponding
> function cr4_set_bits() which takes care of disabling interrupts. But there
> is not. All it does is creating a lot of pointless churn.
> 
>> static __always_inline void setup_smep(struct cpuinfo_x86 *c)
>> static __init int setup_disable_smap(char *arg)
>> static __always_inline void setup_pku(struct cpuinfo_x86 *c)
> 
> Why are you not doing this at the call site around all calls which fiddle
> with cr4, i.e. in identify_cpu() ?
> 
> identify_cpu() is called from two places:
> 
>    identify_boot_cpu() and identify_secondary_cpu()
> 
> identify_secondary_cpu is called with interrupts disabled anyway and there
> is no reason why we can't enforce interrupts being disabled around
> identify_cpu() completely.
> 
> But if we actually do the right thing, i.e. having cr4_set_bit() and
> cr4_set_bit_irqsoff() all of this churn goes away magically.
> 
> Then the only place which needs to be changed is the context switch because
> here interrupts are already disabled and we really care about performance.
> 
>> @@ -293,7 +303,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>> 	}
>> 
>> 	if ((tifp ^ tifn) & _TIF_NOTSC)
>> -		cr4_toggle_bits(X86_CR4_TSD);
>> +		cr4_toggle_bits_irqs_off(X86_CR4_TSD);

You make a good point. I will add cr4_set_bit(). I will leave identify_cpu()
as is, since it is rather hard to maintain code that enables/disables irqs
at one point and rely on these operations at a completely different place.
As you said, it is less of an issue once cr4_set_bit() and friends are
introduced.

Thanks,
Nadav