Re: [Xen-devel] [PATCH 2/2] x86/VMX: don't needlessly install VMFUNC emulation hook

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.12.16 at 14:47, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/12/16 08:37, Jan Beulich wrote:
>> Instead of checking cpu_has_vmx_vmfunc inside the hook, use it to
>> determine whether to install the hook in the first place.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I am not so sure about this.
> 
> vmfunc is reachable in the instruction emulator on hardware which 
> doesn't support vmfunc, and there is explicit provision for using vmfunc 
> 0 via hypercall on hardware lacking vmfunc support.
> 
> Given that the #VE part of altp2m is always emulated architecturally, I 
> think there is an argument to be made for also emulating EPTP switching 
> architecturally as well.

I don't understand this argumentation: Without the patch, the
hook function checks !cpu_has_vmx_vmfunc (and fails otherwise);
with the patch the hook isn't being put in place when
!cpu_has_vmx_vmfunc, and failure occurs in hvmemul_vmfunc().
I admit there's the difference in error codes, but we could
certainly make hvmemul_vmfunc() return EXCEPTION when
there's no hook.

Jan


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