linux-kernel - Re: [PATCH v3 2/2] powerpc/irq: inline call_do_irq() and call_do

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Message-ID: <a7a46d1b-873c-1267-b8ca-615a7fabfd6b@c-s.fr>
Date:   Thu, 19 Sep 2019 07:23:18 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 2/2] powerpc/irq: inline call_do_irq() and
 call_do_softirq()



Le 18/09/2019 à 18:39, Segher Boessenkool a écrit :
> Hi Christophe,
> 
> On Wed, Sep 18, 2019 at 03:48:20PM +0000, Christophe Leroy wrote:
>> call_do_irq() and call_do_softirq() are quite similar on PPC32 and
>> PPC64 and are simple enough to be worth inlining.
>>
>> Inlining them avoids an mflr/mtlr pair plus a save/reload on stack.
> 
> But you hardcode the calling sequence in inline asm, which for various
> reasons is not a great idea.
> 
>> +static inline void call_do_irq(struct pt_regs *regs, void *sp)
>> +{
>> +	register unsigned long r3 asm("r3") = (unsigned long)regs;
>> +
>> +	asm volatile(
>> +		"	"PPC_STLU"	1, %2(%1);\n"
>> +		"	mr		1, %1;\n"
>> +		"	bl		%3;\n"
>> +		"	"PPC_LL"	1, 0(1);\n" : "+r"(r3) :
>> +		"b"(sp), "i"(THREAD_SIZE - STACK_FRAME_OVERHEAD), "i"(__do_irq) :
>> +		"lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7",
>> +		"r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12");
>> +}
> 
> I realise the original code had this...  Loading the old stack pointer
> value back from the stack creates a bottleneck (via the store->load
> forwarding it requires).  It could just use
>    addi 1,1,-(%2)
> here, which can also be written as
>    addi 1,1,%n2
> (that is portable to all architectures btw).

No, we switched stack before the bl call, we replaced r1 by r3 after 
saving r1 into r3 stack. Now we have to restore the original r1.

> 
> 
> Please write the  "+r"(r3)  on the next line?  Not on the same line as
> the multi-line template.  This make things more readable.
> 
> 
> I don't know if using functions as an "i" works properly...  It probably
> does, it's just not something that you see often :-)
> 
> 
> What about r2?  Various ABIs handle that differently.  This might make
> it impossible to share implementation between 32-bit and 64-bit for this.
> But we could add it to the clobber list worst case, that will always work.

Isn't r2 non-volatile on all ABIs ?

> 
> 
> So anyway, it looks to me like it will work.  Nice cleanup.  Would be
> better if you could do the call to __do_irq from C code, but maybe we
> cannot have everything ;-)

sparc do it the following way, is there no risk that GCC adds unwanted 
code inbetween that is not aware there the stack pointer has changed ?

void do_softirq_own_stack(void)
{
	void *orig_sp, *sp = softirq_stack[smp_processor_id()];

	sp += THREAD_SIZE - 192 - STACK_BIAS;

	__asm__ __volatile__("mov %%sp, %0\n\t"
			     "mov %1, %%sp"
			     : "=&r" (orig_sp)
			     : "r" (sp));
	__do_softirq();
	__asm__ __volatile__("mov %0, %%sp"
			     : : "r" (orig_sp));
}

If the above is no risk, then can we do the same on powerpc ?

> 
> Reviewed-by: Segher Boessenkool <segher@...nel.crashing.org>

Thanks for the review.

Christophe