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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.12.16 at 15:31, <JBeulich@xxxxxxxx> wrote:
>>>> 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.

And btw., installing altp2m_vcpu_update_vmfunc_ve is as pointless
in the opposite case, do it bailing early when !cpu_has_vmx_vmfunc.
I guess I'll do both changes for a v2.

Jan


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