linux-kernel - Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable

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Message-ID: <2063b592f82f680edf61dad575f7c092d11d8ba3.camel@redhat.com>
Date:   Tue, 29 Sep 2020 09:39:46 -0400
From:   Qian Cai <cai@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Mohammed Gamal <mgamal@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-next@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address
 space support user-configurable

On Tue, 2020-09-29 at 14:26 +0200, Paolo Bonzini wrote:
> On 29/09/20 13:59, Qian Cai wrote:
> > WARN_ON_ONCE(!allow_smaller_maxphyaddr);
> > 
> > I noticed the origin patch did not have this WARN_ON_ONCE(), but the
> > mainline
> > commit b96e6506c2ea ("KVM: x86: VMX: Make smaller physical guest address
> > space
> > support user-configurable") does have it for some reasons.
> 
> Because that part of the code should not be reached.  The exception
> bitmap is set up with
> 
>         if (!vmx_need_pf_intercept(vcpu))
>                 eb &= ~(1u << PF_VECTOR);
> 
> where
> 
> static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
> {
>         if (!enable_ept)
>                 return true;
> 
>         return allow_smaller_maxphyaddr &&
> 		 cpuid_maxphyaddr(vcpu) < boot_cpu_data.x86_phys_bits;
> }
> 
> We shouldn't get here if "enable_ept && !allow_smaller_maxphyaddr",
> which implies vmx_need_pf_intercept(vcpu) == false.  So the warning is
> genuine; I've sent a patch.

Care to provide a link to the patch? Just curious.