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

["Maciej S. Szmigiero" <mail@maciej.szmigiero.name>]

On 2.04.2022 03:08, Sean Christopherson wrote:
> From: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> 
> 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@oracle.com>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   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