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 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
