Re: [Xen-devel] kexec: Clear notes during setup

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/05/12 16:25, Jan Beulich wrote:
>>>> On 04.05.12 at 17:19, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 04/05/12 13:51, Jan Beulich wrote:
>>>>>> On 04.05.12 at 13:42, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> I know that xen-unstable is on a feature freeze, and this is not
>>>> strictly a bugfix (yet; see below), but as it is safe and designed to
>>>> help clarity in the case of a crash, so I request that it be considered
>>>> for inclusion.
>>>>
>>>> I have constructed an artificial case where the information reported in
>>>> 1 per-cpu crash note was stale by using low_crashinfo mode, crashing
>>>> Xen, allowing it to reboot, offlining a CPU then re-crashing Xen.  This
>>>> leaves stale register state written into the offlined CPU crash
>>>> information.  In this case, the information was stale but correct, due
>>>> to the predictable nature of the Xen crash path from the 'C' debug key,
>>>> but there is no guarantee that in the case of a real crash, the same
>>>> will still be true.
>>> Apart from the missing blanks in the for() statement (which could as
>>> well be a simple memset() afaict),
>>> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> I personally would think that this can go in as a bug fix.
>>>
>>> Jan
>> How would you format the for loop differently? (Not that I mind - just
>> so I know for next time)
>         for ( i = 0; i < (crash_heap_size >> PAGE_SHIFT); ++i )

Ok - refreshed the patch as such

>
>> As for clear_page vs memset - clear_page is faster, and liable to be
>> conditionally tuned more in the future.
> Certainly, but does this matter here?
>
> Jan
>

crash_heap_size scales linearly with the number of PCPUs on the system,
so very large boxes might start noticing a difference in boot speed. 
(Probably not in the grand scheme of things, but as we are explicitly
allocating pages, it makes sense to clear them as pages)

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com

Attachment: kexec-clear-notes.patch
Description: Text Data

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