linux-kernel - Re: [PATCH v3 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

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Message-Id: <4DD05B91-13B3-4842-A215-C8DC6F34F219@amacapital.net>
Date:   Tue, 30 Mar 2021 09:37:30 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>,
        "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD


> On Mar 30, 2021, at 8:14 AM, Sean Christopherson <seanjc@...gle.com> wrote:
> 
> On Mon, Mar 29, 2021, Andy Lutomirski wrote:
>> 
>>>> On Mar 29, 2021, at 7:04 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>>> 
>>> 
>>>> 
>>>>> No, if these instructions take a #VE then they were executed at CPL=0.  MONITOR
>>>>> and MWAIT will #UD without VM-Exit->#VE.  Same for WBINVD, s/#UD/#GP.
>>>> 
>>>> Dare I ask about XSETBV?
>>> 
>>> XGETBV does not cause a #VE, it just works normally. The guest has full
>>> AVX capabilities.
>>> 
>> 
>> X *SET* BV
> 
> Heh, XSETBV also works normally, relative to the features enumerated in CPUID.
> XSAVES/XRSTORS support is fixed to '1' in the virtual CPU model.  A subset of
> the features managed by XSAVE can be hidden by the VMM, but attempting to enable
> unsupported features will #GP (either from hardware or injected by TDX Module),
> not #VE.

Normally in non-root mode means that every XSETBV results in a VM exit and, IIUC, there’s a buglet in that this happens even if CPL==3.  Does something special happen in TDX or does the exit get reflected back to the guest as a #VE?