From: Paolo Bonzini <pbonzini@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>,
Sean Christopherson <seanjc@google.com>
Cc: Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02
Date: Wed, 26 Jan 2022 17:05:30 +0100 [thread overview]
Message-ID: <053bb241-ea71-abf8-262b-7b452dc49d37@redhat.com> (raw)
In-Reply-To: <87r18uh4of.fsf@redhat.com>
On 1/26/22 16:56, Vitaly Kuznetsov wrote:
>> - 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!).
>
Well, that's why the WARN never happens. The idea is that if shadow
VMCS _virtualization_ (not emulation, i.e. running L2 VMREAD/VMWRITE
without even a vmexit to L0) was supported, then you would need a
non-NULL shadow_vmcs in vmx->vmcs02.
Regarding the patch, the old WARN was messy but it was also trying to
avoid a NULL pointer dereference in the caller.
What about:
if (WARN_ON(loaded_vmcs->shadow_vmcs))
return loaded_vmcs->shadow_vmcs;
/* Go ahead anyway. */
WARN_ON(loaded_vmcs != &vmx->vmcs01);
?
Paolo
next prev parent reply other threads:[~2022-01-26 16:05 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-25 22:05 [PATCH] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02 Sean Christopherson
2022-01-26 15:56 ` Vitaly Kuznetsov
2022-01-26 16:05 ` Paolo Bonzini [this message]
2022-01-26 16:22 ` Sean Christopherson
2022-01-26 17:05 ` Paolo Bonzini
2022-01-26 16:27 ` Vitaly Kuznetsov
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