linux-kernel - Re: [PATCH v2] kvm: nVMX: Set nested_run_pending in vmx_set_nested

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Date:   Wed, 8 May 2019 11:13:39 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Aaron Lewis <aaronlewis@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH v2] kvm: nVMX: Set nested_run_pending in
 vmx_set_nested_state after checks complete

On Wed, May 08, 2019 at 10:53:12AM -0700, Aaron Lewis wrote:
> nested_run_pending is also checked in
> nested_vmx_check_vmentry_postreqs
> (https://elixir.bootlin.com/linux/v5.1/source/arch/x86/kvm/vmx/nested.c#L2709)
> so I think the setting needs to be moved to just prior to that call
> with Paolo's rollback along with another for if the prereqs and
> postreqs fail.  I put a patch together below:

Gah, I missed that usage (also, it's now nested_vmx_check_guest_state()).

Side topic, I think the VM_ENTRY_LOAD_BNDCFGS check should be gated by
nested_run_pending, a la the EFER check.'

> ------------------------------------
> 
> nested_run_pending=1 implies we have successfully entered guest mode.
> Move setting from external state in vmx_set_nested_state() until after
> all other checks are complete.
> 
> Signed-off-by: Aaron Lewis <aaronlewis@...gle.com>
> Reviewed-by: Peter Shier <pshier@...gle.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 6401eb7ef19c..cf1f810223d2 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5460,9 +5460,6 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>   if (!(kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE))
>   return 0;
> 
> - vmx->nested.nested_run_pending =
> - !!(kvm_state->flags & KVM_STATE_NESTED_RUN_PENDING);

Alternatively, it might be better to leave nested_run_pending where it
is and instead add a label to handle clearing the flag on error.  IIUC,
the real issue is that nested_run_pending is left set after a failed
vmx_set_nested_state(), not that its shouldn't be set in the shadow
VMCS handling.

Patch attached, though it's completely untested.  The KVM selftests are
broken for me right now, grrr.

> -
>   if (nested_cpu_has_shadow_vmcs(vmcs12) &&
>       vmcs12->vmcs_link_pointer != -1ull) {
>   struct vmcs12 *shadow_vmcs12 = get_shadow_vmcs12(vcpu);
> @@ -5480,14 +5477,21 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>   return -EINVAL;
>   }
> 
> + vmx->nested.nested_run_pending =
> + !!(kvm_state->flags & KVM_STATE_NESTED_RUN_PENDING);
> +
>   if (nested_vmx_check_vmentry_prereqs(vcpu, vmcs12) ||
> -     nested_vmx_check_vmentry_postreqs(vcpu, vmcs12, &exit_qual))
> +     nested_vmx_check_vmentry_postreqs(vcpu, vmcs12, &exit_qual)) {
> +     vmx->nested.nested_run_pending = 0;
>   return -EINVAL;
> + }
> 
>   vmx->nested.dirty_vmcs12 = true;
>   ret = nested_vmx_enter_non_root_mode(vcpu, false);
> - if (ret)
> + if (ret) {
> + vmx->nested.nested_run_pending = 0;
>   return -EINVAL;
> + }
> 
>   return 0;
>  }

View attachment "0001-KVM-nVMX-Clear-nested_run_pending-if-setting-nested-.patch" of type "text/x-diff" (1976 bytes)