diff mbox

kvm: svm: Only propagate next_rip when guest supports it

Message ID 20151007124700.GE28811@8bytes.org (mailing list archive)
State New, archived
Headers show

Commit Message

Joerg Roedel Oct. 7, 2015, 12:47 p.m. UTC 
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).

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:

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(-)

Comments

kernel test robot Oct. 7, 2015, 12:57 p.m. UTC | #1 
Hi Joerg,

[auto build test ERROR on v4.3-rc4 -- if it's inappropriate base, please ignore]

config: i386-randconfig-x009-201540 (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   arch/x86/kvm/svm.c: In function 'nested_svm_vmexit':
>> arch/x86/kvm/svm.c:2369:28: error: 'vcpu' undeclared (first use in this function)
     if (guest_cpuid_has_nrips(vcpu))
                               ^
   arch/x86/kvm/svm.c:2369:28: note: each undeclared identifier is reported only once for each function it appears in

vim +/vcpu +2369 arch/x86/kvm/svm.c

  2363		nested_vmcb->control.exit_code_hi      = vmcb->control.exit_code_hi;
  2364		nested_vmcb->control.exit_info_1       = vmcb->control.exit_info_1;
  2365		nested_vmcb->control.exit_info_2       = vmcb->control.exit_info_2;
  2366		nested_vmcb->control.exit_int_info     = vmcb->control.exit_int_info;
  2367		nested_vmcb->control.exit_int_info_err = vmcb->control.exit_int_info_err;
  2368	
> 2369		if (guest_cpuid_has_nrips(vcpu))
  2370			nested_vmcb->control.next_rip  = vmcb->control.next_rip;
  2371	
  2372		/*

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation
Bandan Das Oct. 7, 2015, 3:48 p.m. UTC | #2 
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 */
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Joerg Roedel Oct. 7, 2015, 4:14 p.m. UTC | #3 
On Wed, Oct 07, 2015 at 11:48:36AM -0400, Bandan Das wrote:
> 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.

Okay, so I think the warning can be removed.

> > +
> > +	if (guest_cpuid_has_nrips(vcpu))
> > +		nested_vmcb->control.next_rip  = vmcb->control.next_rip;

Note that there is a bug here, instead of vcpu it must be &svm->vcpu.
Somehow I missed to at least compile-test this.

Dirk is currently testing whether this (fixed) patch solves the problem
in his setup.

> >  
> >  	/*
> >  	 * 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.

Yeah, probably right. Since we only write guests next_rip when the guest
supports it via cpuid, there is probably no point in resetting it at
vmrun emulation.


	Joerg

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Dirk Mueller Oct. 7, 2015, 5:03 p.m. UTC | #4 
On 07.10.2015 18:14, Joerg Roedel wrote:

> Dirk is currently testing whether this (fixed) patch solves the problem
> in his setup.

Tested-By: Dirk Mueller <dmueller@suse.com>

Works fine here. Thanks!


Greetings,
Dirk
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diff mbox

Patch

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