Re: [PATCH v2 2/3] KVM: x86: force PDPTRs reload on SMM exit

Maxim Levitsky
2021-09-12T10:34:04Z

On Thu, 2021-09-09 at 00:55 +0000, Sean Christopherson wrote:
> On Mon, Aug 23, 2021, Maxim Levitsky wrote:
> > KVM_REQ_GET_NESTED_STATE_PAGES is also used with VM entries that happen
> > on exit from SMM mode, and in this case PDPTRS must be always reloaded.
> >
> > Thanks to Sean Christopherson for pointing this out.
> >
> > Fixes: 0f85722341b0 ("KVM: nVMX: delay loading of PDPTRs to KVM_REQ_GET_NESTED_STATE_PAGES")
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index fada1055f325..4194fbf5e5d6 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7504,6 +7504,13 @@ static int vmx_leave_smm(struct kvm_vcpu *vcpu, const char *smstate)
> > }
> >
> > if (vmx->nested.smm.guest_mode) {
> > +
> > + /* Exit from the SMM to the non root mode also uses
>
> Just "Exit from SMM to non-root mode", i.e. drop the "the".
>
> Multi-line comments should look like:
>
> /*
> * Exit from SMM ...
>
> though oddly checkpatch doesn't complain about that.
>
> That said, for the comment, it'd be more helpful to explain why the PDPTRs should
> not come from userspace. Something like:
>
> /*
> * Always reload the guest's version of the PDPTRs when exiting
> * from SMM to non-root. If KVM_SET_SREGS2 stuffs PDPTRs while
> * SMM is active, that state is specifically for SMM context.
> * On RSM, all guest state is pulled from its architectural
> * location, whatever that may be.
> */
>
> Though typing that makes me wonder if this is fixing the wrong thing. It seems
> like pdptrs_from_userspace shouldn't be set when SMM is active, though I suppose
> there's a potential ordering issue between KVM_SET_SREGS2 and KVM_SET_VCPU_EVENTS.
> Bummer.
>
> > + * the KVM_REQ_GET_NESTED_STATE_PAGES request,
> > + * but in this case the pdptrs must be always reloaded
> > + */
> > + vcpu->arch.pdptrs_from_userspace = false;
> > +
> > ret = nested_vmx_enter_non_root_mode(vcpu, false);
> > if (ret)
> > return ret;
> > --
> > 2.26.3
> >

I went with your suggestion in the patch 3, and dropped this patch.

Thanks!
Best regards,
Maxim Levitsky