linux-kernel - Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

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Date:   Mon, 21 Oct 2019 23:29:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christophe Leroy <christophe.leroy@....fr>
cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        vincenzo.frascino@....com, luto@...nel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

On Mon, 21 Oct 2019, Christophe Leroy wrote:

> This is a tentative to switch powerpc/32 vdso to generic C implementation.
> It will likely not work on 64 bits or even build properly at the moment.
> 
> powerpc is a bit special for VDSO as well as system calls in the
> way that it requires setting CR SO bit which cannot be done in C.
> Therefore, entry/exit and fallback needs to be performed in ASM.
> 
> To allow that, C fallbacks just return -1 and the ASM entry point
> performs the system call when the C function returns -1.
> 
> The performance is rather disappoiting. That's most likely all
> calculation in the C implementation are based on 64 bits math and
> converted to 32 bits at the very end. I guess C implementation should
> use 32 bits math like the assembly VDSO does as of today.

> gettimeofday:    vdso: 750 nsec/call
> 
> gettimeofday:    vdso: 1533 nsec/call

The only real 64bit math which can matter is the 64bit * 32bit multiply,
i.e.

static __always_inline
u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
{
        return ((cycles - last) & mask) * mult;
}

Everything else is trivial add/sub/shift, which should be roughly the same
in ASM.

Can you try to replace that with:

static __always_inline
u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
{
        u64 ret, delta = ((cycles - last) & mask);
        u32 dh, dl;

        dl = delta;
        dh = delta >> 32;

        res = mul_u32_u32(al, mul);
        if (ah)
                res += mul_u32_u32(ah, mul) << 32;

        return res;
}

That's pretty much what __do_get_tspec does in ASM.

Thanks,

	tglx