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

[Jim Mattson]

On Mon, Jan 18, 2021 at 2:22 AM Mohammed Gamal <mgamal@xxxxxxxxxx> wrote:
>
> On Fri, 2021-01-15 at 16:08 -0800, Jim Mattson wrote:
> > On Thu, Sep 3, 2020 at 7:12 AM Mohammed Gamal <mgamal@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> > > ---
> > >  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.

Um...

We do allow it for SVM, if NPT is not enabled. In fact, we set it
unconditionally in that case. See commit 3edd68399dc15 ("KVM: x86: Add
a capability for GUEST_MAXPHYADDR < HOST_MAXPHYADDR support").

Perhaps it should be a module parameter for SVM as well?

And, in any case, it would be nice if the parameter reverted to false
when the kvm_intel module is unloaded.

> Paolo, what do you think?
>
>