* [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check
@ 2021-06-22 7:24 Sean Christopherson
2021-06-22 22:38 ` David Matlack
2021-06-23 13:26 ` Paolo Bonzini
0 siblings, 2 replies; 3+ messages in thread
From: Sean Christopherson @ 2021-06-22 7:24 UTC (permalink / raw)
To: Paolo Bonzini
Cc: Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, kvm, linux-kernel, David Matlack
Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
root. KVM uses a "direct" PAE paging MMU when TDP is disabled and the
guest is running with paging disabled. In that case, root_hpa points at
the pae_root page (of which only 32 bytes are used), not a standard
shadow page, and the WARN fires (a lot).
Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index b981a044ab55..1cae4485b3bc 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
if (WARN_ON(!VALID_PAGE(hpa)))
return false;
+ /*
+ * A NULL shadow page is legal when shadowing a non-paging guest with
+ * PAE paging, as the MMU will be direct with root_hpa pointing at the
+ * pae_root page, not a shadow page.
+ */
sp = to_shadow_page(hpa);
- if (WARN_ON(!sp))
- return false;
-
- return is_tdp_mmu_page(sp) && sp->root_count;
+ return sp && is_tdp_mmu_page(sp) && sp->root_count;
}
#else
static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
--
2.32.0.288.g62a8d224e6-goog
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check
2021-06-22 7:24 [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check Sean Christopherson
@ 2021-06-22 22:38 ` David Matlack
2021-06-23 13:26 ` Paolo Bonzini
1 sibling, 0 replies; 3+ messages in thread
From: David Matlack @ 2021-06-22 22:38 UTC (permalink / raw)
To: Sean Christopherson
Cc: Paolo Bonzini, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, kvm list, LKML
On Tue, Jun 22, 2021 at 12:24 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
> root. KVM uses a "direct" PAE paging MMU when TDP is disabled and the
> guest is running with paging disabled. In that case, root_hpa points at
> the pae_root page (of which only 32 bytes are used), not a standard
> shadow page, and the WARN fires (a lot).
>
> Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
> Cc: David Matlack <dmatlack@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Thanks for the fix. I was able to reproduce the issue by running a
kvm-unit-test with EPT=N. I'll add that to my "pre-send-email"
workflow in the future.
Reviewed-by: David Matlack <dmatlack@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index b981a044ab55..1cae4485b3bc 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
> if (WARN_ON(!VALID_PAGE(hpa)))
> return false;
>
> + /*
> + * A NULL shadow page is legal when shadowing a non-paging guest with
> + * PAE paging, as the MMU will be direct with root_hpa pointing at the
> + * pae_root page, not a shadow page.
> + */
> sp = to_shadow_page(hpa);
> - if (WARN_ON(!sp))
> - return false;
> -
> - return is_tdp_mmu_page(sp) && sp->root_count;
> + return sp && is_tdp_mmu_page(sp) && sp->root_count;
> }
> #else
> static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
> --
> 2.32.0.288.g62a8d224e6-goog
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check
2021-06-22 7:24 [PATCH] KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check Sean Christopherson
2021-06-22 22:38 ` David Matlack
@ 2021-06-23 13:26 ` Paolo Bonzini
1 sibling, 0 replies; 3+ messages in thread
From: Paolo Bonzini @ 2021-06-23 13:26 UTC (permalink / raw)
To: Sean Christopherson
Cc: Vitaly Kuznetsov, Wanpeng Li, Jim Mattson, Joerg Roedel, kvm,
linux-kernel, David Matlack
On 22/06/21 09:24, Sean Christopherson wrote:
> Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
> root. KVM uses a "direct" PAE paging MMU when TDP is disabled and the
> guest is running with paging disabled. In that case, root_hpa points at
> the pae_root page (of which only 32 bytes are used), not a standard
> shadow page, and the WARN fires (a lot).
>
> Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
> Cc: David Matlack <dmatlack@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.h | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
Queued, thanks.
Paolo
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index b981a044ab55..1cae4485b3bc 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
> if (WARN_ON(!VALID_PAGE(hpa)))
> return false;
>
> + /*
> + * A NULL shadow page is legal when shadowing a non-paging guest with
> + * PAE paging, as the MMU will be direct with root_hpa pointing at the
> + * pae_root page, not a shadow page.
> + */
> sp = to_shadow_page(hpa);
> - if (WARN_ON(!sp))
> - return false;
> -
> - return is_tdp_mmu_page(sp) && sp->root_count;
> + return sp && is_tdp_mmu_page(sp) && sp->root_count;
> }
> #else
> static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }
>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-06-23 13:26 ` Paolo Bonzini
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