Re: [XenGT][IGVT-g] DomU pgt_device structure initialization

["Dr. Greg Wettstein" <greg@wind.enjellic.com>]

On Jan 5, 10:04am, Oleksii Kurochko wrote:
} Subject: Re: [Xen-devel] [XenGT][IGVT-g] DomU pgt_device structure initial

> Hey.

Hi Oleksii, I hope this note finds your day going well.

> Strange for me was that I got vmid=0 and gen_type=0, so I decided go
> to i915_gem_vgtbuffer_ioctl and write next at the start: if
> (!xen_initial_domain()) { return -EPERM; } Also same code is in 3.17
> kernel from XenGT-kernel repo.
>
> it seems that there is no more this error now( from vgt_fb_decoder ), BUT
> there is often freeze or very laggy UI in guest.
> 
> What it can be?

It wasn't in this e-mail but I went through the console logs which
were in one of your postings on the IGVT-g list.  I believe you have
your hypervisor configured for synchronous serial console output
(sync_console command-line parameter) and you are directing your dom0
kernel console logging through the Xen provided serial interface.

Setting this option is documented to cause significant latencies.  In
fact there is a warning about this in the Xen console logs when the
hypervisor boots.  Here is the code snippet from
 xen/drivers/char/console.c:console_endboot() which produces the
message:

if ( opt_sync_console )
{
	printk("**********************************************\n");
        printk("******* WARNING: CONSOLE OUTPUT IS SYNCHRONOUS\n");
        printk("******* This option is intended to aid debugging "
               "of Xen by ensuring\n");
        printk("******* that all output is synchronously delivered "
               "on the serial line.\n");
        printk("******* However it can introduce SIGNIFICANT latencies "
               "and affect\n");
        printk("******* timekeeping. It is NOT recommended for "
               "production use!\n");
        printk("**********************************************\n");


It appears as if you were generating significant amounts of kernel log
output which may be at the root of the unacceptable latencies.  I
would start by turning off that option and see if your guest
performance improves.

> With best regards,
>  Oleksii

Good luck with your work.

Greg

}-- End of excerpt from Oleksii Kurochko

As always,
Dr. G.W. Wettstein, Ph.D.   Enjellic Systems Development, LLC.
4206 N. 19th Ave.           Specializing in information infra-structure
Fargo, ND  58102            development.
PH: 701-281-1686
FAX: 701-281-3949           EMAIL: greg@enjellic.com
------------------------------------------------------------------------------
"Sweeny's Law: The length of a progress report is inversely proportional
 to the amount of progress."

--