x86/traps: Re-enable interrupts after reading cr2 in the #PF handler

x86/traps: Re-enable interrupts after reading cr2 in the #PF handler

Hitting a page fault clobbers %cr2, so if a page fault is handled while
handling a previous page fault then %cr2 will hold the address of the
latter fault rather than the former. In particular, if a debug key
handler happens to trigger during #PF and before %cr2 is read, and that
handler itself encounters a #PF, then %cr2 will be corrupt for the outer #PF
handler.

This patch makes the page fault path delay re-enabling IRQs until %cr2
has been read in order to ensure it stays consistent.

A similar argument holds in additional cases, but they happen to be safe:
    * %dr6 inside #DB: Safe because IST exceptions don't re-enable IRQs.
    * MSR_XFD_ERR inside #NM: Safe because AMX isn't used in #NM handler.

While in the area, remove redundant q suffix to a movq in entry.S and
the space after the comma.

Fixes: a4cd20a19073 ("[XEN] 'd' key dumps both host and guest state.")
Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: b06e76db7c35974f1b127762683e7852ca0c8e76
master date: 2024-10-01 09:45:49 +0200