Re: [Xen-devel] [PATCH v4 2/2] x86/Intel: virtualize support for cpuid faulting

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Kyle Huey [mailto:me@xxxxxxxxxxxx]
> Sent: Tuesday, October 18, 2016 2:51 AM
> 
> On HVM guests, the cpuid triggers a vm exit, so we can check the emulated
> faulting state in vmx_do_cpuid and hvmemul_cpuid. A new function,
> hvm_check_cpuid_fault will check if cpuid faulting is enabled and the CPL > 0.
> When it returns true, the cpuid handling functions will inject a GP(0). 
> Notably
> no hardware support for faulting on cpuid is necessary to emulate support with
> an HVM guest.
> 
> On PV guests, hardware support is required so that userspace cpuid will trap
> to Xen. Xen already enables cpuid faulting on supported CPUs for pv guests 
> (that
> aren't the control domain, see the comment in intel_ctxt_switch_levelling).
> Every PV guest cpuid will trap via a GP(0) to emulate_privileged_op (via
> do_general_protection). Once there we simply decline to emulate cpuid if the
> CPL > 0 and faulting is enabled, leaving the GP(0) for the guest kernel to
> handle.
> 
> Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>, assuming you'll fix cosmetic
comments in next version.

Thanks
Kevin

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