linux-kernel - Re: [RFC PATCH 28/35] KVM: X86: Update kvm_skip_emulated

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Date:   Mon, 14 Sep 2020 14:51:45 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 28/35] KVM: X86: Update
 kvm_skip_emulated_instruction() for an SEV-ES guest

On Mon, Sep 14, 2020 at 03:15:42PM -0500, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@....com>
> 
> The register state for an SEV-ES guest is encrypted so the value of the
> RIP cannot be updated. For an automatic exit, the RIP will be advanced
> as necessary. For a non-automatic exit, it is up to the #VC handler in
> the guest to advance the RIP.
> 
> Add support to skip any RIP updates in kvm_skip_emulated_instruction()
> for an SEV-ES guest.

Is there a reason this can't be handled in svm?  E.g. can KVM be reworked
to effectively split the emulation logic so that it's a bug for KVM to end
up trying to modify RIP?

Also, patch 06 modifies SVM's skip_emulated_instruction() to skip the RIP
update, but keeps the "svm_set_interrupt_shadow(vcpu, 0)" logic.  Seems like
either that change or this one is wrong.

> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/kvm/x86.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 23564d02d158..1dbdca607511 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6874,13 +6874,17 @@ static int kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu)
>  
>  int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  {
> -	unsigned long rflags = kvm_x86_ops.get_rflags(vcpu);
> +	unsigned long rflags;
>  	int r;
>  
>  	r = kvm_x86_ops.skip_emulated_instruction(vcpu);
>  	if (unlikely(!r))
>  		return 0;
>  
> +	if (vcpu->arch.vmsa_encrypted)
> +		return 1;
> +
> +	rflags = kvm_x86_ops.get_rflags(vcpu);
>  	/*
>  	 * rflags is the old, "raw" value of the flags.  The new value has
>  	 * not been saved yet.
> -- 
> 2.28.0
>