Syslinux - Mar 2016 - "Tick-counting" vs "Tick-less" timekeeping issues on VMs emulating BIOS PCs

Before 6.X Syslinux used a "Tick-less" timekeeping approach
implemented in

/core/bios.inc 

6.X now implements a "Tick-counting" strategy (timer interrupt)
implemented in

/core/timer.inc

I think this change presents issues when Sysylinux runs on Virtual Machines 

emulating a BIOS environment as they cannot correctly emulate the timer
interrupt;

see VMware's pdf:

https://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf

At the moment I'm seeing timing issues on TFTP transfers (lwIP depends
on the new interrupt based timer). I have consistently detected 

multiple Requests, double ACKs, etc. All these problems seem to be

sourced on the now unreliable timeouts.
I have used TFTP on the same (BIOS VM) scenarios with 3.X and TFTP 

timing was always rock solid.


Sorry if this was discussed before, if not I'd like to ask (Peter) if 

there is any reason preventing us from going back to the "Tick-less" 

approach. Thanks.


Best
Patrick

On 03/01/16 07:21, Patrick Masotta via Syslinux wrote:
> 
> At the moment I'm seeing timing issues on TFTP transfers (lwIP depends
> on the new interrupt based timer). I have consistently detected 
> 
> multiple Requests, double ACKs, etc. All these problems seem to be
> 
> sourced on the now unreliable timeouts.
> I have used TFTP on the same (BIOS VM) scenarios with 3.X and TFTP 
> 
> timing was always rock solid.
> 
> 
> Sorry if this was discussed before, if not I'd like to ask (Peter) if 
> there is any reason preventing us from going back to the
"Tick-less"
> approach. Thanks.
> 
There are quite a few; primarily we simply cannot make lwIP
(lpxelinux.0) work without it.  However, pxelinux.0 should not depend on
this.

	-hpa

On 03/01/16 21:11, H. Peter Anvin via Syslinux wrote:
> On 03/01/16 07:21, Patrick Masotta via Syslinux wrote:
>>
>> At the moment I'm seeing timing issues on TFTP transfers (lwIP
depends
>> on the new interrupt based timer). I have consistently detected 
>>
>> multiple Requests, double ACKs, etc. All these problems seem to be
>>
>> sourced on the now unreliable timeouts.
>> I have used TFTP on the same (BIOS VM) scenarios with 3.X and TFTP 
>>
>> timing was always rock solid.
>>
>>
>> Sorry if this was discussed before, if not I'd like to ask (Peter)
if
>> there is any reason preventing us from going back to the
"Tick-less"
>> approach. Thanks.
>>
> 
> There are quite a few; primarily we simply cannot make lwIP
> (lpxelinux.0) work without it.  However, pxelinux.0 should not depend on
> this.
> 
Now, tickful timekeeping is wasteful, but this is a bootloader, and
functionality is the main concern.  The "tickless" version still
depended on the BIOS tick.

	-hpa