Re: [Qemu-devel] Hight Processor time of Socket communciation

[Jiahuan Zhang <jiahuanzhang90@gmail.com>]

On 19 April 2017 at 11:15, Peter Maydell <peter.maydell@linaro.org> wrote:

> On 19 April 2017 at 09:56, Jiahuan Zhang <jiahuanzhang90@gmail.com> 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.
>
> Hi, no, I am not using any kernel driver and I only test the guest to host
data transfer.
I need the kernel transparent guest-host communication.
The code is in this way.
/***************************************************************/*
int main() {
const char *filename = "image_set/Snake_River_(5mb).jpg";
 /* get the file size */
 struct stat buf;
 uint32_t zero = stat(filename, &buf);
 if (zero == 0)
  printf("image size = %d \n", buf.st_size);
 else
  printf("stat() failed");
 /* open the file */
 uint32_t fd = open(filename, O_RDONLY);
 if(!fd){
  printf("could not open file.\n");
  close(fd);
  return 0;
 }
/* read file into s */
 while((ret_in = read(fd, &s[0], BUF_SIZE)) > 0){
  write_to_uart(s, ret_in);
 }
}

/*
 * write_to_uart(): write data to serial port
 */
void write_to_uart(char* out, uint32_t writeSize){
 int i;
 for(i=0; i<writeSize; i++){
  *UART1 =(unsigned char)(*(out+i));
 }
}

regards,
Huan


> >> 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
>