linux-kernel - Re: [PATCH v2 01/66] KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED

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Date:   Tue, 3 Mar 2020 15:16:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v2 01/66] KVM: x86: Return -E2BIG when
 KVM_GET_SUPPORTED_CPUID hits max entries

On 03/03/20 00:56, Sean Christopherson wrote:
> (KVM hard caps CPUID 0xD at a single sub-leaf).

Hmm... no it doesn't?

                for (idx = 1, i = 1; idx < 64; ++idx) {
                        u64 mask = ((u64)1 << idx);
                        if (*nent >= maxnent)
                                goto out;

                        do_host_cpuid(&entry[i], function, idx);
                        if (idx == 1) {
                                entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
                                cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
                                entry[i].ebx = 0;
                                if (entry[i].eax & (F(XSAVES)|F(XSAVEC)))
                                        entry[i].ebx =
                                                xstate_required_size(supported,
                                                                     true);
                        } else {
                                if (entry[i].eax == 0 || !(supported & mask))
                                        continue;
                                if (WARN_ON_ONCE(entry[i].ecx & 1))
                                        continue;
                        }
                        entry[i].ecx = 0;
                        entry[i].edx = 0;
                        ++*nent;
                        ++i;
                }

I still think the patch is correct, what matters is that no KVM in
existence supports enough processor features to reach 100 or so subleaves.

Paolo