On 19 April 2017 at 09:56, Jiahuan Zhang <[email protected]> wrote:
> Do you mean that it is reasonable for QEMU emulation consumes high CPU time
> when doing host-guest interaction, since the interaction calls many QEMU
> codes in the background?
> Since my guest app is rather simple and no while() is included, according to
> your words,
> can I conclude that the high processor time is cause by the callbacks for
> guest to host data transfer?
What is happening is that the guest kernel's serial driver
has a loop that (simplified) looks like this:
do {
if (pl011_read(REG_FR) & FR_TXFF)
break; /* fifo full, try again later */
pl011_write(buffer[x], REG_DR); /* send one byte */
x++;
} while (x != len);
This is a lot of guest CPU instructions (and two callouts
to QEMU's device emulation) for every single byte.
>> You will likely get better throughput if you use the 'virt' board
>> where you can use the virtio-serial device which can send
>> data more efficiently.
> Here, can I understand your statement in this way,
> a transmit buffer in the serial device for guest to host data transfer
> may reduce the processor time, and in turn, increase the throughput?
The reason virtio-serial is faster is because the guest
kernel driver can essentially tell QEMU
"the data is in guest memory at address X length L"
and then QEMU takes all that data at once. This is much
more efficient.
thanks
-- PMM
