Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> WARN if KVM attempts to allocate a shadow VMCS for vmcs02. KVM emulates
> VMCS shadowing but doesn't virtualize it, i.e. KVM should never allocate
> a "real" shadow VMCS for L2.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/vmx/nested.c | 22 ++++++++++++----------
> 1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index f235f77cbc03..92ee0d821a06 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4851,18 +4851,20 @@ static struct vmcs *alloc_shadow_vmcs(struct kvm_vcpu *vcpu)
> struct loaded_vmcs *loaded_vmcs = vmx->loaded_vmcs;
>
> /*
> - * We should allocate a shadow vmcs for vmcs01 only when L1
> - * executes VMXON and free it when L1 executes VMXOFF.
> - * As it is invalid to execute VMXON twice, we shouldn't reach
> - * here when vmcs01 already have an allocated shadow vmcs.
> + * KVM allocates a shadow VMCS only when L1 executes VMXON and frees it
> + * when L1 executes VMXOFF or the vCPU is forced out of nested
> + * operation. VMXON faults if the CPU is already post-VMXON, so it
> + * should be impossible to already have an allocated shadow VMCS. KVM
> + * doesn't support virtualization of VMCS shadowing, so vmcs01 should
> + * always be the loaded VMCS.
> */
> - WARN_ON(loaded_vmcs == &vmx->vmcs01 && loaded_vmcs->shadow_vmcs);
> + if (WARN_ON(loaded_vmcs != &vmx->vmcs01 || loaded_vmcs->shadow_vmcs))
> + return loaded_vmcs->shadow_vmcs;
Stupid question: why do we want to care about 'loaded_vmcs' at all,
i.e. why can't we hardcode 'vmx->vmcs01' in alloc_shadow_vmcs()? The
only caller is enter_vmx_operation() and AFAIU 'loaded_vmcs' will always
be pointing to 'vmx->vmcs01' (as enter_vmx_operation() allocates
&vmx->nested.vmcs02 so 'loaded_vmcs' can't point there!).
> +
> + loaded_vmcs->shadow_vmcs = alloc_vmcs(true);
> + if (loaded_vmcs->shadow_vmcs)
> + vmcs_clear(loaded_vmcs->shadow_vmcs);
>
> - if (!loaded_vmcs->shadow_vmcs) {
> - loaded_vmcs->shadow_vmcs = alloc_vmcs(true);
> - if (loaded_vmcs->shadow_vmcs)
> - vmcs_clear(loaded_vmcs->shadow_vmcs);
> - }
> return loaded_vmcs->shadow_vmcs;
> }
>
>
> base-commit: edb9e50dbe18394d0fc9d0494f5b6046fc912d33
--
Vitaly
