linux-kernel - Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

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Date:   Wed, 7 Jun 2023 13:22:34 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, tony.luck@...el.com,
        peterz@...radead.org, tglx@...utronix.de, pbonzini@...hat.com,
        david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ying.huang@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, bagasdotme@...il.com,
        sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

On 6/7/23 13:08, Sean Christopherson wrote:
>>>>>> The current TDX_MODULE_CALL macro handles neither #GP nor #UD.  The
>>>>>> kernel would hit Oops if SEAMCALL were mistakenly made w/o enabling VMX
>>>>>> first.  Architecturally, there is no CPU flag to check whether the CPU
>>>>>> is in VMX operation.  Also, if a BIOS were buggy, it could still report
>>>>>> valid TDX private KeyIDs when TDX actually couldn't be enabled.
>>>>> I'm not sure this is a great justification.  If the BIOS is lying to the
>>>>> OS, we _should_ oops.
>>>>>
>>>>> How else can this happen other than silly kernel bugs.  It's OK to oops
>>>>> in the face of silly kernel bugs.
>>>> TDX KVM + reboot can hit #UD.  On reboot, VMX is disabled (VMXOFF) via
>>>> syscore.shutdown callback.  However, guest TD can be still running to issue
>>>> SEAMCALL resulting in #UD.
>>>>
>>>> Or we can postpone the change and make the TDX KVM patch series carry a patch
>>>> for it.
>>> How does the existing KVM use of VMLAUNCH/VMRESUME avoid that problem?
>> extable. From arch/x86/kvm/vmx/vmenter.S
>>
>> .Lvmresume:
>>         vmresume
>>         jmp .Lvmfail
>>
>> .Lvmlaunch:
>>         vmlaunch
>>         jmp .Lvmfail
>>
>>         _ASM_EXTABLE(.Lvmresume, .Lfixup)
>>         _ASM_EXTABLE(.Lvmlaunch, .Lfixup)
> More specifically, KVM eats faults on VMX and SVM instructions that occur after
> KVM forcefully disables VMX/SVM.

<grumble> That's a *TOTALLY* different argument than the patch makes.

KVM is being a _bit_ nutty here, but I do respect it trying to honor the
"-f".  I have no objections to the SEAMCALL code being nutty in the same
way.

Why do I get the feeling that code is being written without
understanding _why_, despite this being v11?