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AC97 can allocate ~500MB of host RAM
While working with qtest test cases generated via fuzzing with QEMU 2.5.0, I discovered some odd behavior for the AC97 virtual device with qemu-system-i386. If AC97_MIC_ADC_RATE is set to the value of 1, the QEMU process allocates over 500MB of additional host RAM. You probably would not normally notice this on a modern PC, except that I was using a "ulimit" command to restrict the maximum amount of virtual memory allowed for the QEMU process, so the process would crash with a SIGTRAP (signal 5) on the failed memory allocation.
My minimized qtest code to reproduce the issue is:
static void test_crash(void)
{
uint64_t barsize;
dev = get_device();
dev_base[0] = qpci_iomap(dev, 0, &barsize);
dev_base[1] = qpci_iomap(dev, 1, &barsize);
qpci_device_enable(dev);
qpci_io_writew(dev, dev_base[0]+0x32, 0x00000001);
}
I ran a "ulimit -sv 650000" command and then launched the tests/ac97-test binary with this crash test case included in it. I can then see the QEMU process crash on an allocation of 722538464 bytes. I can gradually increase the ulimit memory limit to ~1200000 and then no longer see the issue, hence my estimate of 500 MB of RAM allocated by the device.
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