Re: [SeaBIOS] Reducing SeaBIOS kernel entry time

[Kevin O'Connor]

On Fri, Jul 03, 2015 at 03:12:14PM +0100, Stefan Hajnoczi wrote:
> On Fri, Jul 3, 2015 at 2:13 PM, Kevin O'Connor  wrote:
> > I took a look a month or so ago:
> >
> > http://article.gmane.org/gmane.comp.emulators.kvm.devel/136207
> >
> > Last test I ran I had SeaBIOS down to 16ms (not including the time to
> > deploy or jump to the linux kernel).  More reductions are also
> > possible - the above was just with Kconfig settings.
> >
> > Let me know if you want more info on how I did that.
>
> That sounds interesting.  Do you have a .config or git branch?

See attached SeaBIOS config.  I profiled the results following the
directions at:

http://www.seabios.org/Debugging#Timing_debug_messages

The QEMU command line I used is:

qemu-system-x86_64 -k en-us -snapshot -L test -chardev pipe,path=qemudebugpipe,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios -enable-kvm

To get to these low start times (~16ms on my old AMD system) I
disabled debug messages in SeaBIOS.  (The debug messages themselves
take a few milliseconds.)  So, in order to do profiling via the debug
output, I manually added two small debug port writes (see seabios
patch below).  I end up seeing results like this on my machine:

======= Sat Jul  4 13:36:25 2015 (adjust=0.0us)
00.000: 1
00.014: 2
00.015: 2

As discussed in the thread linked to above, these numbers could be
improved further by making code changes (to seabios and/or qemu).
These tests were just with minimal seabios Kconfig changes.

Finally, note that this SeaBIOS setup is so stripped down that it
can't actually boot an OS.  The goal of the experiment was to time
SeaBIOS to the point where one could add an optimized Linux deployment
scheme to SeaBIOS - such as Paolo's idea of putting a kernel in a
"flash" based memory mapped region.  Thus, the seabios numbers above
would also need to include the cost of the kernel deployment and
launching, as well as the qemu startup costs, in order for them to be
truly meaningful.

-Kevin

--- a/src/boot.c
+++ b/src/boot.c
@@ -714,6 +714,7 @@ do_boot(int seq_nr)
     if (seq_nr >= BEVCount)
         boot_fail();

+    outb('2', 0x402); outb('\n', 0x402);
     // Boot the given BEV type.
     struct bev_s *ie = &BEV[seq_nr];
     switch (ie->type) {
diff --git a/src/post.c b/src/post.c
index 6157b50..9fd5f8f 100644
--- a/src/post.c
+++ b/src/post.c
@@ -329,6 +329,7 @@ handle_post(void)
 {
     if (!CONFIG_QEMU && !CONFIG_COREBOOT)
         return;
+    outb('1', 0x402); outb('\n', 0x402);

serial_debug_preinit();
     debug_banner();