Re: [PATCH 1/8] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02

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On 2.04.2022 03:08, Sean Christopherson wrote:
From: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>

The next_rip field of a VMCB is *not* an output-only field for a VMRUN.
This field value (instead of the saved guest RIP) in used by the CPU for
the return address pushed on stack when injecting a software interrupt or
INT3 or INTO exception.

Make sure this field gets synced from vmcb12 to vmcb02 when entering L2 or
loading a nested state and NRIPS is exposed to L1.  If NRIPS is supported
in hardware but not exposed to L1 (nrips=0 or hidden by userspace), stuff
vmcb02's next_rip from the new L2 RIP to emulate a !NRIPS CPU (which
saves RIP on the stack as-is).

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
  arch/x86/kvm/svm/nested.c | 22 +++++++++++++++++++---
  arch/x86/kvm/svm/svm.h    |  1 +
  2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 73b545278f5f..9a6dc2b38fcf 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -369,6 +369,7 @@ void __nested_copy_vmcb_control_to_cache(struct kvm_vcpu *vcpu,
  	to->nested_ctl          = from->nested_ctl;
  	to->event_inj           = from->event_inj;
  	to->event_inj_err       = from->event_inj_err;
+	to->next_rip            = from->next_rip;
  	to->nested_cr3          = from->nested_cr3;
  	to->virt_ext            = from->virt_ext;
  	to->pause_filter_count  = from->pause_filter_count;
@@ -606,7 +607,8 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
  	}
  }
-static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
+static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
+					  unsigned long vmcb12_rip)
  {
  	u32 int_ctl_vmcb01_bits = V_INTR_MASKING_MASK;
  	u32 int_ctl_vmcb12_bits = V_TPR_MASK | V_IRQ_INJECTION_BITS_MASK;
@@ -660,6 +662,19 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
  	vmcb02->control.event_inj           = svm->nested.ctl.event_inj;
  	vmcb02->control.event_inj_err       = svm->nested.ctl.event_inj_err;
+ /*
+	 * next_rip is consumed on VMRUN as the return address pushed on the
+	 * stack for injected soft exceptions/interrupts.  If nrips is exposed
+	 * to L1, take it verbatim from vmcb12.  If nrips is supported in
+	 * hardware but not exposed to L1, stuff the actual L2 RIP to emulate
+	 * what a nrips=0 CPU would do (L1 is responsible for advancing RIP
+	 * prior to injecting the event).
+	 */
+	if (svm->nrips_enabled)
+		vmcb02->control.next_rip    = svm->nested.ctl.next_rip;
+	else if (boot_cpu_has(X86_FEATURE_NRIPS))
+		vmcb02->control.next_rip    = vmcb12_rip;
+
  	vmcb02->control.virt_ext            = vmcb01->control.virt_ext &
  					      LBR_CTL_ENABLE_MASK;
  	if (svm->lbrv_enabled)
@@ -743,7 +758,7 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa,
  	nested_svm_copy_common_state(svm->vmcb01.ptr, svm->nested.vmcb02.ptr);
svm_switch_vmcb(svm, &svm->nested.vmcb02);
-	nested_vmcb02_prepare_control(svm);
+	nested_vmcb02_prepare_control(svm, vmcb12->save.rip);
  	nested_vmcb02_prepare_save(svm, vmcb12);
ret = nested_svm_load_cr3(&svm->vcpu, svm->nested.save.cr3,
@@ -1422,6 +1437,7 @@ static void nested_copy_vmcb_cache_to_control(struct vmcb_control_area *dst,
  	dst->nested_ctl           = from->nested_ctl;
  	dst->event_inj            = from->event_inj;
  	dst->event_inj_err        = from->event_inj_err;
+	dst->next_rip             = from->next_rip;
  	dst->nested_cr3           = from->nested_cr3;
  	dst->virt_ext              = from->virt_ext;
  	dst->pause_filter_count   = from->pause_filter_count;
@@ -1606,7 +1622,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
  	nested_copy_vmcb_control_to_cache(svm, ctl);
svm_switch_vmcb(svm, &svm->nested.vmcb02);
-	nested_vmcb02_prepare_control(svm);
+	nested_vmcb02_prepare_control(svm, save->rip);

					   ^
I guess this should be "svm->vmcb->save.rip", since
KVM_{GET,SET}_NESTED_STATE "save" field contains vmcb01 data,
not vmcb{0,1}2 (in contrast to the "control" field).


Thanks,
Maciej



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