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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.12.16 at 16:14, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/12/16 14:58, Jan Beulich wrote:
>>>>> 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.
> 
> My argument is that, instead of excluding the hook, the behaviour of the 
> emulation path should be made to function sensibly even on hardware 
> without vmfunc.
> 
> i.e. drop the cpu_has_vmx_vmfunc check and do nothing else.

Ah, I see. I guess I'll leave that to someone having an environment
to test this. The patch's goal was to not change observable behavior.

Jan


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