Re: [Xen-devel] [Patch] time resolution fix.

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 17 Mar 2006, at 16:11, Dong, Eddie wrote:

>         I have a much detail description in the attached slide, hope
> this helpful.
Well, freezing the TSC while a guest is descheduled is not very nice at 
all, but I can imagine it stops you getting time went backwards 
messages if you are also forcibly re-setting the TSC on PIT ticks. :-)
The freezing is I guess why you have the new hook schedule_out(), which 
I'm also not madly keen on. Especially since this must surely be a 
short-term workaround (you don't intend to TSC freeze as long-term 
solution, right?).
>         BTW, due to SMP support and more time resource support (RTC and
> ACPI), we are planning to do some design modification to sync all those
> different kind of time. This patch is mainly for bug fix that exist for
> long time and block SMP effort.
Clearly some effort needs to be applied here. Moving all time emulation 
into Xen itself would be a good start (e.g., strip PIT emulation from 
qemu-dm).
And the new support should be HVM generic, since there are no 
differences in time handling between VMX and SVM that should require 
(much) vendor-specific handling I think. If there are, or extra vendor 
support appears in future (e.g., I'd like to see guest TSC rate 
control, as I've said a few times before ;-) ), then we can add vendor 
hooks in later.
In summary, I'm not sure about this patch. I feel that if I take it I'm 
encouraging 'onward and upward' development without spending the time 
to make sure fundamental abstractions like time are designed and 
implemented soundly.
 -- Keir


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