[Xen-devel] Mapping active GDT

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

I am looking into GDT remap series [0] which crashes PV guests and it
seems that the problem lies in the fact that we cannot establish new
mapping to an already existing GDT.

The mapping is created by

+static inline void setup_fixmap_gdt(int cpu)
+{
+       __set_fixmap(get_cpu_gdt_ro_index(cpu),
+                    __pa(get_cpu_gdt_rw(cpu)), PAGE_KERNEL);
+}

with get_cpu_gdt_rw(cpu) being the current GDT pointer. This results in

(XEN) mm.c:2570:d94v0 Bad type (saw 5400000000000001 != exp
7000000000000000) for mfn 1538fb (pfn 3e809)
(XEN) mm.c:1022:d94v0 Could not get page type PGT_writable_page
(XEN) mm.c:1074:d94v0 Error getting mfn 1538fb (pfn 3e809) from L1 entry
80000001538fb063 for l1e_owner=94, pg_owner=94

(after a small change to xen_set_fixmap(), which I think was missing)

Before I try to come up with a fix I wanted to check here to see if this
(not being able to map active GDT) is indeed the case.

-boris


[0]
https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg00869.html


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