Re: [RFC 2/7] KVM: VMX: Expose IA32_PKRS MSR

[Chenyi Qiang]

On 8/14/2020 1:31 AM, Jim Mattson wrote:
> On Wed, Aug 12, 2020 at 10:42 PM Chenyi Qiang <chenyi.qiang@xxxxxxxxx> wrote:
> > On 8/13/2020 5:21 AM, Jim Mattson wrote:
> > > On Fri, Aug 7, 2020 at 1:46 AM Chenyi Qiang <chenyi.qiang@xxxxxxxxx> wrote:
> > > > Protection Keys for Supervisor Pages (PKS) uses IA32_PKRS MSR (PKRS) at
> > > > index 0x6E1 to allow software to manage supervisor protection key
> > > > rights. For performance consideration, PKRS intercept will be disabled
> > > > so that the guest can access the PKRS without VM exits.
> > > > PKS introduces dedicated control fields in VMCS to switch PKRS, which
> > > > only does the retore part. In addition, every VM exit saves PKRS into
> > > > the guest-state area in VMCS, while VM enter won't save the host value
> > > > due to the expectation that the host won't change the MSR often. Update
> > > > the host's value in VMCS manually if the MSR has been changed by the
> > > > kernel since the last time the VMCS was run.
> > > > The function get_current_pkrs() in arch/x86/mm/pkeys.c exports the
> > > > per-cpu variable pkrs_cache to avoid frequent rdmsr of PKRS.
> > > >
> > > > Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> > > > ---
> > >
> > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > > index 11e4df560018..df2c2e733549 100644
> > > > --- a/arch/x86/kvm/vmx/nested.c
> > > > +++ b/arch/x86/kvm/vmx/nested.c
> > > > @@ -289,6 +289,7 @@ static void vmx_sync_vmcs_host_state(struct vcpu_vmx *vmx,
> > > >           dest->ds_sel = src->ds_sel;
> > > >           dest->es_sel = src->es_sel;
> > > >    #endif
> > > > +       dest->pkrs = src->pkrs;
> > >
> > > Why isn't this (and other PKRS code) inside the #ifdef CONFIG_X86_64?
> > > PKRS isn't usable outside of long mode, is it?
> >
> > Yes, I'm also thinking about whether to put all pks code into
> > CONFIG_X86_64. The kernel implementation also wrap its pks code inside
> > CONFIG_ARCH_HAS_SUPERVISOR_PKEYS which has dependency with CONFIG_X86_64.
> > However, maybe this can help when host kernel disable PKS but the guest
> > enable it. What do you think about this?
>
> I see no problem in exposing PKRS to the guest even if the host
> doesn't have CONFIG_ARCH_HAS_SUPERVISOR_PKEYS.

Yes, but I would prefer to keep it outside CONFIG_X86_64. PKS code has 
several code blocks and putting them under x86_64 may end up being a 
mess. In addition, PKU KVM related code isn't under CONFIG_X86_64 as 
well. So, is it really necessary to put inside?