Re: [Qemu-devel] [kvm-unit-tests PATCHv4 3/3] arm: pmu: Add CPI checking

[Andrew Jones <drjones@redhat.com>]

On Mon, Oct 19, 2015 at 11:44:30AM -0400, Christopher Covington wrote:
> Hi Drew,
> 
> I appreciate your feedback on these patches.
> 
> On 10/18/2015 02:28 PM, Andrew Jones wrote:
> 
> >> --- a/arm/pmu.c
> >> +++ b/arm/pmu.c
> >> @@ -37,6 +37,18 @@ static inline unsigned long get_pmccntr(void)
> >>  	asm volatile("mrc p15, 0, %0, c9, c13, 0" : "=r" (cycles));
> >>  	return cycles;
> >>  }
> >> +
> >> +static inline void loop(int i, uint32_t pmcr)
> >> +{
> >> +	uint32_t z = 0;
> >> +
> >> +	asm volatile(
> >> +		"	mcr p15, 0, %[pmcr], c9, c12, 0\n"
> >> +		"	1: subs %[i], %[i], #1\n"
> >> +		"	bgt 1b\n"
> >> +		"	mcr p15, 0, %[z], c9, c12, 0\n"
> >> +	: [i] "+r" (i) : [pmcr] "r" (pmcr), [z] "r" (z) : "cc");
> > 
> > Assembly is always ugly, but we can do a bit better formatting with tabs
> > 
> > 	asm volatile(
> > 	"	mcr	p15, 0, %[pmcr], c9, c12, 0\n"
> > 	"1:	subs	%[i], %[i], #1\n"
> > 	"	bgt	1b\n"
> > 	"	mcr	p15, 0, %[z], c9, c12, 0\n"
> > 	: [i] "+r" (i)
> > 	: [pmcr] "r" (pmcr), [z] "r" (z)
> > 	: "cc");
> > 
> > Actually it can be even cleaner because you already created set_pmcr()
> > 
> > 	set_pmcr(pmcr);
> > 
> > 	asm volatile(
> > 	"1:	subs	%0, %0, #1\n"
> > 	"	bgt     1b\n"
> > 	: "+r" (i) : : "cc");
> > 
> > 	set_pmcr(0);
> 
> Is there any way to ensure that the compiler won't for example put a `mov rd,
> #0` between the `bgt 1b` and the `mcr <pmcr>, rn`?

You're right. We need to keep the clearing in the asm here in order to
make sure don't add instructions in between.

> 
> >> @@ -125,12 +147,79 @@ static bool check_cycles_increase(void)
> >>  	return true;
> >>  }
> >>  
> >> -int main(void)
> >> +/*
> >> + * Execute a known number of guest instructions. Only odd instruction counts
> >> + * greater than or equal to 3 are supported by the in-line assembly code. The
> > 
> > Not all odd counts, right? But rather all multiples of 3? IIUC this is because
> > the loop is two instructions (sub + branch), and then the clearing of the pmcr
> > register counts as the 3rd?
> 
> Clearing the PMCR doesn't happen as part of the loop, but as part of the loop
> exit or epilogue.
> 
> total_instrs = iteration_count * loop_instrs + eipilogue_instrs
> total_instrs = iteration_count * 2 + 1

Ah yeah, that makes sense.

Thanks,
drew

> 
> Thanks,
> Christopher Covington
> 
> -- 
> Qualcomm Innovation Center, Inc.
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