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

[Jim Mattson]

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.