Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable

[Mohammed Gamal <mgamal@redhat.com>]

On Fri, 2021-01-15 at 16:08 -0800, Jim Mattson wrote:
> On Thu, Sep 3, 2020 at 7:12 AM Mohammed Gamal <mgamal@redhat.com>
> wrote:
> > 
> > This patch exposes allow_smaller_maxphyaddr to the user as a module
> > parameter.
> > 
> > Since smaller physical address spaces are only supported on VMX,
> > the parameter
> > is only exposed in the kvm_intel module.
> > Modifications to VMX page fault and EPT violation handling will
> > depend on whether
> > that parameter is enabled.
> > 
> > Also disable support by default, and let the user decide if they
> > want to enable
> > it.
> > 
> > Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 15 ++++++---------
> >  arch/x86/kvm/vmx/vmx.h |  3 +++
> >  arch/x86/kvm/x86.c     |  2 +-
> >  3 files changed, 10 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 819c185adf09..dc778c7b5a06 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -129,6 +129,9 @@ static bool __read_mostly
> > enable_preemption_timer = 1;
> >  module_param_named(preemption_timer, enable_preemption_timer,
> > bool, S_IRUGO);
> >  #endif
> > 
> > +extern bool __read_mostly allow_smaller_maxphyaddr;
> 
> Since this variable is in the kvm module rather than the kvm_intel
> module, its current setting is preserved across "rmmod kvm_intel;
> modprobe kvm_intel." That is, if set to true, it doesn't revert to
> false after "rmmod kvm_intel." Is that the intended behavior?
> 

IIRC, this is because this setting was indeed not intended to be just
VMX-specific, but since AMD has an issue with PTE accessed-bits being
set by hardware and thus we can't yet enable this feature on it, it
might make sense to move the variable to the kvm_intel module for now.

Paolo, what do you think?