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

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Message-ID: <ebcd39a5-364f-c4ac-f8c7-41057a3d84be@redhat.com>
Date:   Tue, 29 Sep 2020 14:26:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Qian Cai <cai@...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 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.

Paolo