linux-kernel - Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED

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Message-ID: <36289f8c-85e1-4bb9-aff3-648ead059cd0@intel.com>
Date: Fri, 13 Sep 2024 11:54:55 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org,
 kai.huang@...el.com, isaku.yamahata@...il.com,
 tony.lindgren@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from
 KVM_GET_SUPPORTED_CPUID

On 9/13/2024 2:41 AM, Sean Christopherson wrote:
>>> CET might be a bad example because it looks like it's controlled by TDCS.XFAM, but
>>> presumably there are other CPUID-based features that would actively enable some
>>> feature for a TDX VM.
>> XFAM is controlled by userspace though, not KVM, so we've got no
>> control on that either.
> I assume it's plain text though?  I.e. whatever ioctl() sets TDCS.XFAM can be
> rejected by KVM if it attempts to enable unsupported features?

yes. XFAM is validated by KVM actually in this series.

KVM reports supported_xfam via KVM_TDX_CAPABILITIES and userspace sets 
XFAM via ioctl(KVM_TDX_VM_INIT). If userspace sets any bits beyond the 
supported_xfam, KVM returns -EINVAL.

The same for attributes.

> I don't expect that we'll want KVM to gatekeep many, if any features, but I do
> think we should require explicit enabling in KVM whenever possible, even if the
> enabling is boring and largely ceremonial.
+1