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

From: Jim Mattson <jmattson@google.com>
To: Chenyi Qiang <chenyi.qiang@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Joerg Roedel <joro@8bytes.org>, Xiaoyao Li <xiaoyao.li@intel.com>,
	kvm list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/7] KVM: VMX: Expose IA32_PKRS MSR
Date: Tue, 18 Aug 2020 11:23:47 -0700	[thread overview]
Message-ID: <CALMp9eSAkzGPp4zPVakypR1McSJtJ1x4j1zAAj1sM1bHxd01zg@mail.gmail.com> (raw)
In-Reply-To: <268b0ee4-e56f-981c-c03e-6dca8a4e99da@intel.com>

On Tue, Aug 18, 2020 at 12:28 AM Chenyi Qiang <chenyi.qiang@intel.com> wrote:
>
>
>
> On 8/14/2020 1:31 AM, Jim Mattson wrote:
> > On Wed, Aug 12, 2020 at 10:42 PM Chenyi Qiang <chenyi.qiang@intel.com> wrote:
> >>
> >>
> >>
> >> On 8/13/2020 5:21 AM, Jim Mattson wrote:
> >>> On Fri, Aug 7, 2020 at 1:46 AM Chenyi Qiang <chenyi.qiang@intel.com> 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@intel.com>
> >>>> ---
> >>>
> >>>> 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?

I'll let someone who actually cares about the i386 build answer that question.