Re: [PATCH] kvm: svm: Only propagate next_rip when guest supports it

[Bandan Das <bsd@redhat.com>]

Joerg Roedel <joro@8bytes.org> writes:

> On Wed, Oct 07, 2015 at 01:03:35PM +0200, Joerg Roedel wrote:
>> But we don't care if L1 writes something into its own next_rip, as we
>> never read this value from its VMCB. We only copy the next_rip value we
>> get from our shadow-vmcb to it on an emulated vmexit. So I still don't
>> understand what triggers the reported problem or why the WARN_ON is
>> necessary.
>
> Okay, I think I have an idea now. I talked a bit with Dirk and the
> WARN_ON triggers in the guest, and not on the host. This makes a lot
> more sense.
>
> In nested-svm we always copy the next_rip from the shadow-vmcb to the
> guests vmcb, even when the nrips bit in cpuid is not set for the guest.
> This obviously triggers the WARN_ON() in the L1 KVM (I still don't
> understand why the WARN_ON was introduced in the first place).

Ok, understood now. The warn_on would trigger in L1 only if it has
decided to disable nrips for some reason as was the case here. So,
my reasoning behind putting the warning was incorrect. 

> So the right fix is to only copy next_rip to the guests vmcb when its
> cpuid indicates that next_rip is supported there, like in this patch:

Yep, agreed.

> From 019afc60507618b8e44e0c67d5ea2d850d88c9dd Mon Sep 17 00:00:00 2001
> From: Joerg Roedel <jroedel@suse.de>
> Date: Wed, 7 Oct 2015 13:38:19 +0200
> Subject: [PATCH] kvm: svm: Only propagate next_rip when guest supports it
>
> Currently we always write the next_rip of the shadow vmcb to
> the guests vmcb when we emulate a vmexit. This could confuse
> the guest when its cpuid indicated no support for the
> next_rip feature.
>
> Fix this by only propagating next_rip if the guest actually
> supports it.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/kvm/cpuid.h | 21 +++++++++++++++++++++
>  arch/x86/kvm/svm.c   |  7 ++++++-
>  2 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index dd05b9c..effca1f 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -133,4 +133,25 @@ static inline bool guest_cpuid_has_mpx(struct kvm_vcpu *vcpu)
>  	best = kvm_find_cpuid_entry(vcpu, 7, 0);
>  	return best && (best->ebx & bit(X86_FEATURE_MPX));
>  }
> +
> +/*
> + * NRIPS is provided through cpuidfn 0x8000000a.edx bit 3
> + */
> +#define BIT_NRIPS	3
> +
> +static inline bool guest_cpuid_has_nrips(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry(vcpu, 0x8000000a, 0);
> +
> +	/*
> +	 * NRIPS is a scattered cpuid feature, so we can't use
> +	 * X86_FEATURE_NRIPS here (X86_FEATURE_NRIPS would be bit
> +	 * position 8, not 3).
> +	 */
> +	return best && (best->edx & bit(BIT_NRIPS));
> +}
> +#undef BIT_NRIPS
> +
>  #endif
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 94b7d15..e1a8824 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2459,7 +2459,9 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
>  	nested_vmcb->control.exit_info_2       = vmcb->control.exit_info_2;
>  	nested_vmcb->control.exit_int_info     = vmcb->control.exit_int_info;
>  	nested_vmcb->control.exit_int_info_err = vmcb->control.exit_int_info_err;
> -	nested_vmcb->control.next_rip          = vmcb->control.next_rip;
> +
> +	if (guest_cpuid_has_nrips(vcpu))
> +		nested_vmcb->control.next_rip  = vmcb->control.next_rip;
>  
>  	/*
>  	 * If we emulate a VMRUN/#VMEXIT in the same host #vmexit cycle we have
> @@ -2714,6 +2716,9 @@ static bool nested_svm_vmrun(struct vcpu_svm *svm)
>  	svm->vmcb->control.event_inj = nested_vmcb->control.event_inj;
>  	svm->vmcb->control.event_inj_err = nested_vmcb->control.event_inj_err;
>  
> +	/* Clear next_rip, as real hardware would do */
> +	nested_vmcb->control.next_rip = 0;
> +

Why do we need this ? And are you sure this is what real hardware does ?
I couldn't find anything in the spec.

>  	nested_svm_unmap(page);
>  
>  	/* Enter Guest-Mode */