Re: [RFC 7/7] KVM: VMX: Enable PKS for nested VM

From: Chenyi Qiang
Date: Thu Aug 13 2020 - 00:54:37 EST

On 8/11/2020 8:05 AM, Jim Mattson wrote:
On Fri, Aug 7, 2020 at 1:47 AM Chenyi Qiang <chenyi.qiang@xxxxxxxxx> wrote:

PKS MSR passes through guest directly. Configure the MSR to match the
L0/L1 settings so that nested VM runs PKS properly.

Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
arch/x86/kvm/vmx/nested.c | 32 ++++++++++++++++++++++++++++++++
arch/x86/kvm/vmx/vmcs12.c | 2 ++
arch/x86/kvm/vmx/vmcs12.h | 6 +++++-
arch/x86/kvm/vmx/vmx.c | 10 ++++++++++
arch/x86/kvm/vmx/vmx.h | 1 +
5 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index df2c2e733549..1f9823d21ecd 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -647,6 +647,12 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,

+ if (!msr_write_intercepted_l01(vcpu, MSR_IA32_PKRS))
+ nested_vmx_disable_intercept_for_msr(
+ msr_bitmap_l1, msr_bitmap_l0,

What if L1 intercepts only *reads* of MSR_IA32_PKRS?

kvm_vcpu_unmap(vcpu, &to_vmx(vcpu)->nested.msr_bitmap_map, false);

return true;

@@ -2509,6 +2519,11 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
if (kvm_mpx_supported() && (!vmx->nested.nested_run_pending ||
!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
vmcs_write64(GUEST_BNDCFGS, vmx->nested.vmcs01_guest_bndcfgs);
+ if (kvm_cpu_cap_has(X86_FEATURE_PKS) &&

Is the above check superfluous? I would assume that the L1 guest can't
set VM_ENTRY_LOAD_IA32_PKRS unless this is true.

I enforce this check to ensure vmcs_write to the Guest_IA32_PKRS without error. if deleted, vmcs_write to GUEST_IA32_PKRS may executed when PKS is unsupported.

+ (!vmx->nested.nested_run_pending ||
+ !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PKRS)))
+ vmcs_write64(GUEST_IA32_PKRS, vmx->nested.vmcs01_guest_pkrs);

This doesn't seem right to me. On the target of a live migration, with
L2 active at the time the snapshot was taken (i.e.,
vmx->nested.nested_run_pending=0), it looks like we're going to try to
overwrite the current L2 PKRS value with L1's PKRS value (except that
in this situation, vmx->nested.vmcs01_guest_pkrs should actually be
0). Am I missing something?

We overwrite the L2 PKRS with L1's value when L2 doesn't support PKS. Because the L1's VM_ENTRY_LOAD_IA32_PKRS is off, we need to migrate L1's PKRS to L2.

vmx_set_rflags(vcpu, vmcs12->guest_rflags);

/* EXCEPTION_BITMAP and CR0_GUEST_HOST_MASK should basically be the

@@ -3916,6 +3943,8 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
if (kvm_mpx_supported())
vmcs12->guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
+ if (kvm_cpu_cap_has(X86_FEATURE_PKS))

Shouldn't we be checking to see if the *virtual* CPU supports PKS
before writing anything into vmcs12->guest_ia32_pkrs?

Yes, It's reasonable.

+ vmcs12->guest_ia32_pkrs = vmcs_read64(GUEST_IA32_PKRS);

vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;