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Subject: Re: [PATCH v3 6/8] KVM:VMX: Load Guest CET via VMCS when CET is
 enabled in Guest
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Yang Weijiang <weijiang.yang@intel.com>
Cc:     rkrcmar@redhat.com, jmattson@google.com,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mst@redhat.com,
        yu-cheng.yu@intel.com, Zhang Yi Z <yi.z.zhang@linux.intel.com>,
        wei.w.wang@intel.com, weijiang.yang@inte.com
References: <20190225132716.6982-1-weijiang.yang@intel.com>
 <20190225132716.6982-7-weijiang.yang@intel.com>
 <20190228161715.GF6166@linux.intel.com>
 <20190228083844.GC12006@local-michael-cet-test.sh.intel.com>
 <20190301145819.GC22584@linux.intel.com>
 <20190303122608.GA32013@local-michael-cet-test.sh.intel.com>
 <20190304184307.GC17120@linux.intel.com>
 <20190304095640.GA3576@local-michael-cet-test.sh.intel.com>
 <20190305031202.GI17120@linux.intel.com>
 <20190304123655.GB4185@local-michael-cet-test.sh.intel.com>
 <20190308162835.GB2528@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <9e80935f-b572-8210-6ea1-a582b3d05f88@redhat.com>
Date:   Fri, 8 Mar 2019 17:36:50 +0100
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Sender: linux-kernel-owner@vger.kernel.org
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On 08/03/19 17:28, Sean Christopherson wrote:
> On Mon, Mar 04, 2019 at 08:36:55PM +0800, Yang Weijiang wrote:
>> On Mon, Mar 04, 2019 at 07:12:02PM -0800, Sean Christopherson wrote:
>>> On Mon, Mar 04, 2019 at 05:56:40PM +0800, Yang Weijiang wrote:
>>>> Cannot agree with you more!
>>>> This is some design limitation, but from my point of view, once vmm
>>>> exposes CET capability to guest via CPUID, it grants the guest kernel freedom to choose
>>>> which features to be enabled, we don't need to add extra constraints on
>>>> the usage.
>>>
>>> But if KVM allows SHSTK and IBT to be toggled independently then the VMM
>>> has only exposed SHSTK or IBT, not CET as whole.
>>>
>>> Even if SHSTK and IBT are bundled together the guest still has to opt-in
>>> to enabling each feature.  I don't see what we gain by pretending that
>>> SHSTK/IBT can be individually exposed to the guest, and on the flip side
>>> doing so creates a virtualization hole.
>> you almost convinced me ;-), maybe I'll make the feature as a bundle in
>> next release after check with kernel team. BTW, what do you mean by
>> saying "create a virtualization hole"? Is it what you stated in above
>> reply?
> 
> By "virtualization hole" I mean the guest would be able to use a feature
> that the virtual CPU model says isn't supported.

I think it's okay to leave the hole and leave it to userspace to forbid
enabling only one of the bits.

Paolo

> After rereading the XSS architecture, there's a marginally less crappy
> option for handling XRSTOR as we could use the XSS_EXIT_BITMAP to
> intercept XRSTOR if SHSTK != IBT and the guest is restoring CET state,
> e.g. to ensure the guest isn't setting IA32_PL*_SSP if !SHSTK and isn't
> setting bits that are effectively reserved in IA32_U_CET.
> 
> But practically speaking that'd be the same as intercepting XRSTORS
> unconditionally when the guest is using CET, i.e. it's still going to
> tank the performance of a guest that uses CET+XSAVES/XRSTORS.
>