Date:   Tue, 28 Dec 2021 23:38:52 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Jing Liu <jing2.liu@intel.com>
Cc:     x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, pbonzini@redhat.com, corbet@lwn.net,
        shuah@kernel.org, jun.nakajima@intel.com, kevin.tian@intel.com,
        jing2.liu@linux.intel.com, guang.zeng@intel.com,
        wei.w.wang@intel.com, yang.zhong@intel.com
Subject: Re: [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at
 KVM_SET_CPUID2
Message-ID: <YcugDN+9d8t4r9bV@google.com>
References: <20211222124052.644626-1-jing2.liu@intel.com>
 <20211222124052.644626-6-jing2.liu@intel.com>
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On Wed, Dec 22, 2021, Jing Liu wrote:
> Guest xstate permissions should be set by userspace VMM before vcpu
> creation. Extend KVM_SET_CPUID2 to verify that every feature reported
> in CPUID[0xD] has proper permission set.

Why?  Nothing in the changelog, code, or comments explains why KVM _needs_ to prevent
userspace from advertising bogus features to the guest.  E.g. the virtual address
width check exists because _KVM_ will do the wrong thing given a width other than 48
or 57, and explicity says as much in a comment.