linux-kernel - Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Date: Fri, 17 May 2024 16:27:51 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Dave Hansen
	<dave.hansen@...el.com>
CC: <rick.p.edgecombe@...el.com>, <pbonzini@...hat.com>, <x86@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peterz@...radead.org>, <chao.gao@...el.com>, <mlevitsk@...hat.com>,
	<john.allen@....com>
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On 5/17/2024 12:58 AM, Sean Christopherson wrote:
> On Thu, May 16, 2024, Dave Hansen wrote:
>> On 5/16/24 07:39, Sean Christopherson wrote:
>>>> We synced the issue internally, and got conclusion that KVM should honor host
>>>> IBT config.  In this case IBT bit in boot_cpu_data should be honored.  With
>>>> this policy, it can avoid CPUID confusion to guest side due to host ibt=off
>>>> config.
>>> What was the reasoning?  CPUID confusion is a weak justification, e.g. it's not
>>> like the guest has visibility into the host kernel, and raw CPUID will still show
>>> IBT support in the host.
>> I'm basically arguing for the path of least resistance (at least to start).
>>
>> We should just do what takes the least amount of code for now that
>> results in mostly sane behavior, then debate about making it perfect later.
>>
>> In other words, let's say the place we'd *IDEALLY* end up is that guests
>> can have any random FPU state which is disconnected from the host.  But
>> the reality, for now, is that the host needs to have XFEATURE_CET_USER
>> set in order to pass it into the guest and that means keeping
>> X86_FEATURE_SHSTK set.
>>
>> If you want guest XFEATURE_CET_USER, you must have host
>> X86_FEATURE_SHSTK ... for now.
> Ah, because fpu__init_system_xstate() will clear XFEATURE_CET_USER via the
> X86_FEATURE_SHSTK connection in xsave_cpuid_features.
>
> Please put something to that effect in the changelog.  "this literally won't work
> (without more changes)" is very different than us making a largely arbitrary
> decision.

So I need to remove the trick here for guest IBT, right?

Side topic:
When X86_FEATURE_SHSTK and X86_FEATURE_IBT (no ibt=off set) are available on host side, then host XFEATURE_CET_USER is enabled. In this case, we still *need* below patch: https://lore.kernel.org/all/20240219074733.122080-3-weijiang.yang@intel.com/ to correctly enable XFEATURE_CET_USER in *guest kernel*, because VMM userspace can enable IBT alone for guest by -cpu host -shstk, am I right?