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SubjectXen PV breakage after IRQ stack code refactoring

(Can't find the original patch in my mailbox)

This hunk from 1d3e53e8624a ("x86/entry/64: Refactor IRQ stacks and make
them NMI-safe")

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a9a8027..0d4483a 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -447,6 +447,59 @@ ENTRY(irq_entries_start)

+ pushfq
+ testl $X86_EFLAGS_IF, (%rsp)
+ jz .Lokay_\@
+ ud2
+ addq $8, %rsp

makes Xen PV guests somewhat unhappy because IF flag will be set.

I was hoping to use ALTERNATIVE instruction but when we hit this for the
first time we haven't rewritten instructions yet. Moving check_bugs() a
bit higher helps but because this is common code I don't know how well
it will work on other architectures (and, in fact, whether it is even
safe on x86 in general, although that can be verified).

Another option is to also add a parameter to DEBUG_ENTRY_ASSERT_IRQS_OFF
(or to ENTER_IRQ_STACK) from xen_do_hypervisor_callback (which is where
the failure happens) but this looks pretty fragile in that it assumes
that xen_do_hypervisor_callback is the only place where we use this
codepath before alt instructions are set.
Any other suggestions?


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