Re: [Xen-devel] [PATCH V7 3/5] x86/altp2m: fix display frozen when switching to a new view early

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 11/20/18 11:12 AM, Jan Beulich wrote:
>>>> On 19.11.18 at 18:26, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> When an new altp2m view is created very early on guest boot, the
>> display will freeze (although the guest will run normally). This
>> may also happen on resizing the display. The reason is the way
>> Xen currently (mis)handles logdirty VGA: it intentionally
>> misconfigures VGA pages so that they will fault.
>>
>> The problem is that it only does this in the host p2m. Once we
>> switch to a new altp2m, the misconfigured entries will no longer
>> fault, so the display will not be updated.
>>
>> This patch:
>> * updates ept_handle_misconfig() to use the active altp2m instead
>>   of the hostp2m;
>> * modifies p2m_change_entry_type_global(),
>>   p2m_memory_type_changed(), p2m_change_type_range() and
>>   p2m_finish_type_change() to propagate their changes to all
>>   valid altp2ms.
>>
>> With the introduction of altp2m fields in p2m_memory_type_changed()
>> the whole function has been put under CONFIG_HVM.
>>
>> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>> Suggested-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> 
> Judging from George's earlier analysis I wonder whether the patch
> ordering is correct: I've got the impression that the patch here should
> be last in the series, for it to be correct and efficient in all cases.
> 
> Furthermore (minor, but this appears to recur), may I ask that you
> switch around tags like the above ones in the future? As previously
> expressed (in other contexts) I'm of the opinion that the sequence
> of tags should represent the flow of events, and a bug report as
> much as a suggestion of a change can't have happened before the
> writing of a patch.

It's quite true, hence my question here:

https://lists.xenproject.org/archives/html/xen-devel/2018-11/msg02044.html

It has also occured to me that applying this patch before George's (and
_not_ applying his afterwards) will produce code that will pass all the
compilation tests but will still crash the hypervisor when altp2m gets
activated on a guest.

If George doesn't mind, I can try to figure out a way to have them
either applied before the final patch.

As for the tag order, sorry about that - I simply didn't know that was
the convention. It's of course no problem to switch them.


Thanks,
Razvan

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