[SeaBIOS] [PATCH] qemu: avoid debug prints if debugcon is not enabled

[Stefano Garzarella]

On Tue, Dec 11, 2018 at 4:08 AM Kevin O'Connor <kevin at koconnor.net> wrote:
>
> On Sun, Dec 02, 2018 at 02:10:13PM +0100, Stefano Garzarella wrote:
> > In order to speed up the boot phase, we can check the QEMU
> > debugcon device, and disable the writes if it is not recognized.
> >
> > This patch allow us to save around 10 msec (time measured
> > between SeaBIOS entry point and "linuxboot" entry point)
> > when CONFIG_DEBUG_LEVEL=1 and debugcon is not enabled.
>
> Thanks.  In order to properly track reboots, the HaveRunPost variable
> must always be the first global variable to be modified.  So, the qemu
> debug init must come later in the boot.  How about the patch below
> instead?

Thanks for the explanation! It looks good!

Cheers,
Stefano

>
> -Kevin
>
>
> commit 75b42835134553c96f113e5014072c0caf99d092 (HEAD -> master)
> Author: Stefano Garzarella <sgarzare at redhat.com>
> Date:   Sun Dec 2 14:10:13 2018 +0100
>
>     qemu: avoid debug prints if debugcon is not enabled
>
>     In order to speed up the boot phase, we can check the QEMU
>     debugcon device, and disable the writes if it is not recognized.
>
>     This patch allow us to save around 10 msec (time measured
>     between SeaBIOS entry point and "linuxboot" entry point)
>     when CONFIG_DEBUG_LEVEL=1 and debugcon is not enabled.
>
>     Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
>     Signed-off-by: Kevin O'Connor <kevin at koconnor.net>
>
> diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
> index 0770c47..4fcd8f5 100644
> --- a/src/fw/paravirt.c
> +++ b/src/fw/paravirt.c
> @@ -75,6 +75,9 @@ static void qemu_detect(void)
>      if (!CONFIG_QEMU_HARDWARE)
>          return;
>
> +    // Setup QEMU debug output port
> +    qemu_debug_preinit();
> +
>      // check northbridge @ 00:00.0
>      u16 v = pci_config_readw(0, PCI_VENDOR_ID);
>      if (v == 0x0000 || v == 0xffff)
> diff --git a/src/fw/paravirt.h b/src/fw/paravirt.h
> index a14d83e..f7e1d4c 100644
> --- a/src/fw/paravirt.h
> +++ b/src/fw/paravirt.h
> @@ -49,6 +49,9 @@ static inline int runningOnKVM(void) {
>  // QEMU_CFG_DMA ID bit
>  #define QEMU_CFG_VERSION_DMA    2
>
> +// QEMU debugcon read value
> +#define QEMU_DEBUGCON_READBACK  0xe9
> +
>  int qemu_cfg_enabled(void);
>  int qemu_cfg_dma_enabled(void);
>  void qemu_preinit(void);
> diff --git a/src/hw/serialio.c b/src/hw/serialio.c
> index 319a85c..3163344 100644
> --- a/src/hw/serialio.c
> +++ b/src/hw/serialio.c
> @@ -103,11 +103,23 @@ serial_debug_flush(void)
>
>  u16 DebugOutputPort VARFSEG = 0x402;
>
> +void
> +qemu_debug_preinit(void)
> +{
> +    /* Check if the QEMU debug output port is active */
> +    if (CONFIG_DEBUG_IO &&
> +        inb(GET_GLOBAL(DebugOutputPort)) != QEMU_DEBUGCON_READBACK)
> +        DebugOutputPort = 0;
> +}
> +
>  // Write a character to the special debugging port.
>  void
>  qemu_debug_putc(char c)
>  {
> -    if (CONFIG_DEBUG_IO && runningOnQEMU())
> +    if (!CONFIG_DEBUG_IO || !runningOnQEMU())
> +        return;
> +    u16 port = GET_GLOBAL(DebugOutputPort);
> +    if (port)
>          // Send character to debug port.
> -        outb(c, GET_GLOBAL(DebugOutputPort));
> +        outb(c, port);
>  }
> diff --git a/src/hw/serialio.h b/src/hw/serialio.h
> index 88296fe..81fed30 100644
> --- a/src/hw/serialio.h
> +++ b/src/hw/serialio.h
> @@ -24,6 +24,7 @@ void serial_debug_preinit(void);
>  void serial_debug_putc(char c);
>  void serial_debug_flush(void);
>  extern u16 DebugOutputPort;
> +void qemu_debug_preinit(void);
>  void qemu_debug_putc(char c);
>
>  #endif // serialio.h



-- 
Stefano Garzarella
Red Hat