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Re: [Xen-devel] [PATCH v2 4/5] x86: Port the basic alternative mechanism from Linux to Xen

On 29/05/2014 10:28, Wu, Feng wrote:

8 @@ void __init noreturn __start_xen(unsigned long
       if ( cpu_has_fsgsbase )

+    alternative_instructions();
Given this ordering, it might be cleaner to have an
ASSERT(!local_irq_enabled()) in the top of alternative_instructions(),
and forgo the local_irq_save/restore() in text_poke_early().

If you can move this higher up before enabling MCEs in CR4, it might be
slightly more resilient.

MCE bit in CR4 is set in identify_cpu() --> mcheck_init() --> 
set_in_cr4(X86_CR4_MCE), but
apply_alternatives() needs boot_cpu_data.x86_capability being ready, since it 
calls boot_cpu_has().
If we put alternative_instructions() before enabling MCEs in CR4, which place 
do you suggest? Thanks!


One option would be to temporarily disable it in cr4 at the same point that NMIs are nopped out, in the same way as temporarily disabling CR4.SMAP when building dom0.

At the end of the day, an MCE will certainly result in a crash, but at least it wouldn't be from a weird fault because some of the codepath in the MCE handler was midway through being patched.


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