linux-kernel - Re: [PATCH 1/5] KVM: nSVM: Sync next

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Date:   Fri, 1 Apr 2022 18:32:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jon Grimm <Jon.Grimm@....com>,
        David Kaplan <David.Kaplan@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Liam Merwick <liam.merwick@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02

On Thu, Mar 10, 2022, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.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.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> ---
>  arch/x86/kvm/svm/nested.c | 4 ++++
>  arch/x86/kvm/svm/svm.h    | 1 +
>  2 files changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index d736ec6514ca..9656f0d6815c 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -366,6 +366,7 @@ 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;
> @@ -638,6 +639,8 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
>  	svm->vmcb->control.int_state           = svm->nested.ctl.int_state;
>  	svm->vmcb->control.event_inj           = svm->nested.ctl.event_inj;
>  	svm->vmcb->control.event_inj_err       = svm->nested.ctl.event_inj_err;
> +	/* The return address pushed on stack by the CPU for some injected events */
> +	svm->vmcb->control.next_rip            = svm->nested.ctl.next_rip;

This needs to be gated by nrips being enabled _and_ exposed to L1, i.e.

	if (svm->nrips_enabled)
		vmcb02->control.next_rip    = svm->nested.ctl.next_rip;

though I don't see any reason to add the condition to the copy to/from cache flows.

>  	if (!nested_vmcb_needs_vls_intercept(svm))
>  		svm->vmcb->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
> @@ -1348,6 +1351,7 @@ 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;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 93502d2a52ce..f757400fc933 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -138,6 +138,7 @@ struct vmcb_ctrl_area_cached {
>  	u64 nested_ctl;
>  	u32 event_inj;
>  	u32 event_inj_err;
> +	u64 next_rip;
>  	u64 nested_cr3;
>  	u64 virt_ext;
>  	u32 clean;

I don't know why this struct has

	u8 reserved_sw[32];

but presumably it's for padding, i.e. probably should be reduced to 24 bytes.