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Subject: Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2}
 after KVM_RUN is broken
To:     Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
References: <20210622175739.3610207-1-seanjc@google.com>
 <20210622175739.3610207-8-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <f031b6bc-c98d-8e46-34ac-79e540674a55@redhat.com>
Date:   Wed, 23 Jun 2021 16:16:47 +0200
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On 22/06/21 19:56, Sean Christopherson wrote:
> +	/*
> +	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> +	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> +	 * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> +	 * faults due to reusing SPs/SPTEs.  Alert userspace, but otherwise
> +	 * sweep the problem under the rug.
> +	 *
> +	 * KVM's horrific CPUID ABI makes the problem all but impossible to
> +	 * solve, as correctly handling multiple vCPU models (with respect to
> +	 * paging and physical address properties) in a single VM would require
> +	 * tracking all relevant CPUID information in kvm_mmu_page_role.  That
> +	 * is very undesirable as it would double the memory requirements for
> +	 * gfn_track (see struct kvm_mmu_page_role comments), and in practice
> +	 * no sane VMM mucks with the core vCPU model on the fly.
> +	 */
> +	if (vcpu->arch.last_vmentry_cpu != -1)
> +		pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n");

Let's make this even stronger and promise to break it in 5.16.

Paolo