Re: [Qemu-devel] regression in timer code?

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Max Filippov [mailto:jcmvbkbc@gmail.com]
> the commit b39e3f34c9de7ead6a11a74aa2de78baf41d81a7
> ("icount: fixed saving/restoring of icount warp timers") has changed
> something that made timers test for target/xtensa unstable.
> Specifically ccount_write case in the tests/tcg/xtensa/test_timer.S
> now fails for me about half of the times it is run. Given that it is run
> under -icount I guess this is a bug. Could you please take a look?
> 
> The minimal test case is available here:
>   http://jcmvbkbc.spb.ru/~dumb/tmp/201803131306/test_timer.tst
> It is run as
>   qemu-system-xtensa -M sim -cpu dc232b -nographic -semihosting
> -icount 7  -kernel ./test_timer.tst

I investigated your test and concluded the following.
First, update_ccount is inaccurate opration because of the division
with the remainder:
    env->sregs[CCOUNT] = env->ccount_base +
        (uint32_t)((now - env->time_base) *
                   env->config->clock_freq_khz / 1000000);

Therefore, the following sequence in the test may give different result depending
of the actual value of "now" variable.
    rsr     a3, ccount
    rsr     a4, ccount
    sub     a4, a4, a3

Consider the code:

test ccount_write
    rsr     a3, ccount
    rsr     a4, ccount
    sub     a4, a4, a3 ; usually 1, but sometimes 2 because of rounding
    movi    a2, 0x12345678
    wsr     a2, ccount
    esync
    rsr     a3, ccount
    sub     a3, a3, a2 ; usually 3 (esync + yield + rsr), but sometimes 4 because of rounding
    slli    a4, a4, 2  ; 4 or 8
    assert  ltu, a3, a4 ; (3 or 4) < (4 or 8) ?
test_end

Therefore in some cases we get a4=4 and a3=4 that forces the test to fail.

Pavel Dovgalyuk