Re: [PATCH v5 08/13] KVM: selftests: Test Intel PMU architectural events on gp counters

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 26, 2023, Mingwei Zhang wrote:
> On Thu, Oct 26, 2023, Sean Christopherson wrote:
> > Heh, already did this too.  Though I'm not entirely sure it's more readable.  It's
> > definitely more precise and featured :-)
> > 
> Oh dear, this is challenging to my rusty inline assembly skills :)
> 
> > #define GUEST_MEASURE_EVENT(_msr, _value, clflush, FEP)				\
> > do {										\
> > 	__asm__ __volatile__("wrmsr\n\t"					\
> > 			     clflush "\n\t"					\
> > 			     "mfence\n\t"					\
> > 			     "1: mov $" __stringify(NUM_BRANCHES) ", %%ecx\n\t"	\
> > 			     FEP "loop .\n\t"					\
> > 			     FEP "mov %%edi, %%ecx\n\t"				\
> > 			     FEP "xor %%eax, %%eax\n\t"				\
> > 			     FEP "xor %%edx, %%edx\n\t"				\
> > 			     "wrmsr\n\t"					\
> > 			     : "+c"((int){_msr})				\
> isn't it NUM_BRANCHES?

Nope.  It's hard to see because this doesn't provide the usage, but @_msr is an
MSR index that is consumed by the first "wrmsr", i.e. this blob relies on the
compiler to preload ECX, EAX, and EDX for WRMSR.  NUM_BRANCHES is manually loaded
into ECX after WRMSR (WRMSR and LOOP both hardcode consuming ECX).

Ha!  I can actually drop the "+c" clobbering trick since ECX is restored to its
input value before the asm blob finishes.  EDI is also loaded with _@msr so that
it can be quickly reloaded into ECX for the WRMSR to disable the event.

The purpose of doing both WRMSRs in assembly is to ensure the compiler doesn't
insert _any_ code into the measured sequence, e.g. so that a random Jcc doesn't
throw off instructions retired.

> > 			     : "a"((uint32_t)_value), "d"(_value >> 32),	\
> > 			       "D"(_msr)					\
> > 	);									\
> > } while (0)
> >
> 
> do we need this label '1:' in the above code? It does not seems to be
> used anywhere within the code.

It's used by the caller as the target for CLFLUSH{,OPT}.

	if (this_cpu_has(X86_FEATURE_CLFLUSHOPT))				\
		GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflushopt 1f", FEP);	\
	else if (this_cpu_has(X86_FEATURE_CLFLUSH))				\
		GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflushopt 1f", FEP);	\
	else									\
		GUEST_MEASURE_EVENT(_ctrl_msr, _value, "nop", FEP);		\
> 
> why is clflush needed here?

As suggested by Jim, it allows verifying LLC references and misses by forcing
the CPU to evict the cache line that holds the MOV at 1: (and likely holds most
of the asm blob).