lkml.org 
[lkml]   [2013]   [May]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen_ticks
Hi Konrad,

On 05/06/2013 10:35 AM, Konrad Rzeszutek Wilk wrote:
>>> e.g. if a VCPU sets a timer for NOW+5, but 3 are stolen in the middle it
>>> would not make sense (from the guests PoV) for NOW'==NOW+2 at the point
>>> where the timer goes off. Nor does it make sense to require that the
>>> guest actually be running for 5 before injecting the timer because that
>>> would mean real time elapsed time for the timer would be 5+3 in the case
>>> where 3 are stolen.
>>
>> This is a bit of an aside, but I think that hiding time spent at higher
>> privilege levels can be a quite sensible approach to timekeeping in a
>> virtualized environment, but I understand that it's not the approach taken
>> with Xen, and as you pointed out above, adjusting the Virtual Offset Register
>> by itself isn't enough to implement that approach.
>
> This is the approach taken by Xen and KVM. Look in CONFIG_PARAVIRT_CLOCK for
> implementation. In the user-space, the entry in 'top' of "stolen" (%st)
> is for this exact value.

I may have been unclear with my terms, sorry. When I refer to time being
"hidden", I mean that kernel level software (supervisor mode, EL1) cannot
detect the passage of that time at all. I don't know whether this would really
work, but I imagine one might be able to get close with the current
virtualization facilities for ARM.

Am I correct in interpreting that what you're referring to is the deployment
of paravirtualization code that ensures (observable) "stolen" time is factored
into kernel decision-making?

Thanks,
Christopher

--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by the Linux Foundation.


\
 
 \ /
  Last update: 2013-05-07 19:01    [W:0.102 / U:44.304 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site