RE: [patch 3/3] x86/fpu: Make FPU protection more robust

[David Laight]

> -----Original Message-----
> From: Jason A. Donenfeld <Jason@xxxxxxxxx>
> Sent: 05 May 2022 12:36
> To: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Borislav Petkov
> <bp@xxxxxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; Filipe Manana
> <fdmanana@xxxxxxxx>; linux-crypto@xxxxxxxxxxxxxxx
> Subject: Re: [patch 3/3] x86/fpu: Make FPU protection more robust
> 
> On Thu, May 5, 2022 at 1:34 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
> >
> > ...
> > > +     cycles_t end, start = get_cycles();
> > >       blake2s_update(&input_pool.hash, in, nbytes);
> > > +     end = get_cycles();
> >
> > If get_cycles() is rdtsc then that gives meaningless numbers.
> > The cpu clock frequency will change on you.
> >
> > You can use one of the performance counters to get an actual
> 
> Indeed. In the process of wiring up rdpmc now.

I've used this before now.
But the loop getting the pmc value in non-deterministic.
So I sometimes remove it.
Also you need to add a serialising instruction - otherwise
the pmc get read before the code you are measuring
actually finishes.

I've used similar code to measure iterations on the ip checksum code.
Can show how many bytes/clock that achieves in its inner loop.
Which can match what you might expect the instructions to generate.

	David

static inline unsigned int rdpmc(unsigned int counter)
{
        unsigned int low, high;

        asm volatile("rdpmc" : "=a" (low), "=d" (high) : "c" (counter));

        // return low bits, counter might to 32 or 40 bits wide.
        return low;
}

static inline unsigned int rdtsc(void)
{
        unsigned int low, high;

        asm volatile("rdtsc" : "=a" (low), "=d" (high));

        return low;
}

unsigned int read_cpu_cycles(void)
{
        static struct perf_event_attr perf_attr = {
                .type = PERF_TYPE_HARDWARE,
                .config = PERF_COUNT_HW_CPU_CYCLES,
                .pinned = 1,
        };
        static __thread struct perf_event_mmap_page *pc;
        static int pmc_failed;

        unsigned int seq, idx, count;

        if (pmc_failed)
                return rdtsc();

        if (!pc) {
                int perf_fd;
                perf_fd = syscall(__NR_perf_event_open, &perf_attr, 0, -1, -1, 0);
                if (perf_fd < 0) {
                        pmc_failed = 1;
                        return rdtsc();
                }
                pc = mmap(NULL, sysconf(_SC_PAGESIZE), PROT_READ, MAP_SHARED, perf_fd, 0);
                close(perf_fd);
                if (pc == MAP_FAILED) {
                        pmc_failed = 1;
                        return rdtsc();
                }
        }

        do {
                seq = pc->lock;
                asm volatile("":::"memory");
                idx = pc->index;
                if (!idx) //  || !pc->cap_user_rdpmc)
                        return 0;
                count = pc->offset + rdpmc(idx - 1);
                asm volatile("":::"memory");
        } while (pc->lock != seq);

        return count;
}

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