Re: [kvm-unit-tests PATCH] lib/arm/io: Fix calling getchar() multiple times

["Nicholas Piggin" <npiggin@gmail.com>]

On Sat Feb 17, 2024 at 12:02 AM AEST, Thomas Huth wrote:
> getchar() can currently only be called once on arm since the implementation
> is a little bit too  naïve: After the first character has arrived, the
> data register never gets set to zero again. To properly check whether a
> byte is available, we need to check the "RX fifo empty" on the pl011 UART
> or the "RX data ready" bit on the ns16550a UART instead.
>
> With this proper check in place, we can finally also get rid of the
> ugly assert(count < 16) statement here.
>

Thanks, this seems to work well. But arm64 still behaves strangely
with QEMU migration. It seems some odd corruption around __getchar,
but this turns out not to have anything to do with the MMIO read,
but it's just because the test case generally is spinning there
when a migration happens.

I modified __getchar() to the following, which removes the MMIO
entirely and just waits a number of times called before returning
a getchar, chosen to be enough time for QEMU migration to complete
on my system...

int __getchar(void)
{
        static int i;
        static volatile int locked;
        int ret = -1;

        assert(!locked);
        locked = 1;
        assert(locked);
        cpu_relax();
        assert(locked);

        i++;
        if (i >= 3000000) {
                i = 0;
                ret = 1;
        }

        assert(locked);
        locked = 0;
        assert(!locked);

        return ret;
}

I get asserts on the line right after cpu_relax() on arm64 after a
few migrations on migration selftest.

Without the cpu_relax() it takes longer to hit an assert and it's
usually the first one. I expect this is related to TCG translation
blocks and where it exits back to be migrated. Something is very
strange, I suspect it's QEMU migration bug.

Thanks,
Nick