[Xen-devel] [PATCH] x86/shadow: Drop incorrect diagnostic when shadowing TSS.RSP0

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

During development of the XTF pagewalk tests, I reliably encountered this
message exactly once per run.  It occurs when the first action to touch
TSS.RSP0 is an interrupt/exception taken in userspace, and the processor tries
to push the IRET frame.

Subsequently, OSSTest has demonstrated that it triggers frequently for a
KPTI-enabled kernel.

  (XEN) multi.c:3324:d1v1 write to pagetable during event injection: 
cr2=0xffffad2646687f38, mfn=0x2415a1
  [ 1411.949155] systemd-logind[2683]: New session 73 of user root.
  (XEN) multi.c:3324:d1v1 write to pagetable during event injection: 
cr2=0xffffad264671ff38, mfn=0x240a41
  (XEN) multi.c:3324:d1v1 write to pagetable during event injection: 
cr2=0xffffad2646837f38, mfn=0x2415c5
  (XEN) multi.c:3324:d1v1 write to pagetable during event injection: 
cr2=0xffffad26468a7f38, mfn=0x2414e7
  [ 1442.207473] systemd-logind[2683]: New session 74 of user root.
  [ 1471.452206] systemd-logind[2683]: New session 75 of user root.
  (XEN) multi.c:3324:d1v1 write to pagetable during event injection: 
cr2=0xffffad2646d17f08, mfn=0x2417c5
  [ 1501.698971] systemd-logind[2683]: New session 76 of user root.

The actions performed by the shadow code are correct, and the guest continues
without error, but the emitted error is misleading.  Tweak the comment more
clearly identify why the condition exists, but drop the message.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Tim Deegan <tim@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I could have sworn I posted this patch a while ago, but I can't find any
evidence of actually having done so.  Oh well - better late than never.
---
 xen/arch/x86/mm/shadow/multi.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 1d282c9..460ec80 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -3305,8 +3305,9 @@ static int sh_page_fault(struct vcpu *v,
     {
         /*
          * If we are in the middle of injecting an exception or interrupt then
-         * we should not emulate: it is not the instruction at %eip that caused
-         * the fault. Furthermore it is almost certainly the case the handler
+         * we should not emulate: the fault is a side effect of the processor
+         * trying to push an exception frame onto a stack which has yet to be
+         * shadowed.  Furthermore it is almost certainly the case the handler
          * stack is currently considered to be a page table, so we should
          * unshadow the faulting page before exiting.
          */
@@ -3319,9 +3320,6 @@ static int sh_page_fault(struct vcpu *v,
                 v->arch.paging.last_write_emul_ok = 0;
             }
 #endif
-            gdprintk(XENLOG_DEBUG, "write to pagetable during event "
-                     "injection: cr2=%#lx, mfn=%#lx\n",
-                     va, mfn_x(gmfn));
             sh_remove_shadows(d, gmfn, 0 /* thorough */, 1 /* must succeed */);
             trace_shadow_emulate_other(TRC_SHADOW_EMULATE_UNSHADOW_EVTINJ,
                                        va, gfn);
-- 
2.1.4


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