Re: [Xen-devel] [PATCH V3] vmx/nmi: Do not use self_nmi() in VMEXIT handler

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.02.13 at 15:42, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> ... this must not be done when on the NMI stack (i.e. when the
> NMI was raised while in hypervisor context). Checking for this
> here would be strait forward, but I was really considering to do
> all of this in the assembly exit path, and I was still undecided
> whether we shouldn't follow Linux in skipping softirq processing
> (and hence scheduling) on the way out from an NMI (I don't
> think we'd need to do the same for MCE).

Like this:

x86: skip processing events on the NMI exit path

Otherwise, we may end up in the scheduler, keeping NMIs masked for a
possibly unbounded time (until whenever the next IRET gets executed).

Of course it's open for discussion whether to always use the strait
exit path from handle_ist_exception.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -171,7 +171,7 @@ compat_bad_hypercall:
         jmp  compat_test_all_events
 
 /* %rbx: struct vcpu, interrupts disabled */
-compat_restore_all_guest:
+ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
         RESTORE_ALL adj=8 compat=1
 .Lft0:  iretq
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -635,6 +635,9 @@ ENTRY(early_page_fault)
         jmp   restore_all_xen
         .popsection
 
+ENTRY(nmi)
+        pushq $0
+        movl  $TRAP_nmi,4(%rsp)
 handle_ist_exception:
         SAVE_ALL
         testb $3,UREGS_cs(%rsp)
@@ -649,12 +652,17 @@ handle_ist_exception:
         movzbl UREGS_entry_vector(%rsp),%eax
         leaq  exception_table(%rip),%rdx
         callq *(%rdx,%rax,8)
-        jmp   ret_from_intr
+        cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
+        jne   ret_from_intr
 
-ENTRY(nmi)
-        pushq $0
-        movl  $TRAP_nmi,4(%rsp)
-        jmp   handle_ist_exception
+        /* We want to get strait to the IRET in the NMI exit path. */
+        testb $3,UREGS_cs(%rsp)
+        GET_CURRENT(%rbx)
+        jz    restore_all_xen
+        movq  VCPU_domain(%rbx),%rax
+        testb $1,DOMAIN_is_32bit_pv(%rax)
+        jz    restore_all_guest
+        jmp   compat_restore_all_guest
 
 ENTRY(nmi_crash)
         pushq $0




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