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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

> Jan

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