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Re: [Xen-devel] [PATCH RFC] x86/altp2m: fix display frozen when switching to a new view early
- To: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>, <george.dunlap@xxxxxxxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Wed, 19 Sep 2018 13:44:31 +0100
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- Cc: andrew.cooper3@xxxxxxxxxx, kevin.tian@xxxxxxxxx, wei.liu2@xxxxxxxxxx, jbeulich@xxxxxxxx, jun.nakajima@xxxxxxxxx
- Delivery-date: Wed, 19 Sep 2018 12:44:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 09/19/2018 01:15 PM, George Dunlap wrote:
> On 09/03/2018 09:25 AM, Razvan Cojocaru 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.
>
> Hey Razvan, thanks for doing this, and sorry it's taken so long to respond.
>
>> This patch:
>>
>> * updates ept_handle_misconfig() to use the active altp2m instead
>> of the hostp2m;
>
> This is probably necessary.
>
>> * has p2m_init_altp2m_ept() copy over max_mapped_pfn,
>> logdirty_ranges, global_logdirty, ept.ad and default_access
>> from the hostp2m (the latter more for completeness than for any
>> other reason).
>
> I think this is probably the right approach. These values change
> rarely, but after a misconfig are read repeatedly. So it's probably a
> lot more efficient to propagate changes when they happen, rather than
> trying to keep a single master copy. However...
>
>> We should discuss if just copying over
>> logdirty_ranges (which is a pointer) is sufficient, or if
>> this code requires more synchronization).
>
> It's clearly not sufficient. :-) The logdirty_ranges struct is
> protected by the lock of the p2m structure that contains it; if you
> point to it from a different p2m structure, then you'll have
> inconsistent logging, and you'll have problems if one vcpu is reading
> the structure while another is modifying it.
...and therefore, if we believe that it's more efficient to duplicate
structures than to share it and use a lock, we need to do a deep copy of
the data structure on altp2m creation, and propagate changes as we do
for the other "synced" data.
-George
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