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Date:   Wed, 4 Aug 2021 21:44:58 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter H Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 11/12] x86/tdx: Don't write CSTAR MSR on Intel
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References: <20210804181329.2899708-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20210804181329.2899708-12-sathyanarayanan.kuppuswamy@linux.intel.com>
 <YQrdFLPaUnC8Q5bn@google.com>
 <4c1ee7b9-9941-fdc4-73f5-3d2ef0530556@linux.intel.com>
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On Wed, Aug 04, 2021, Kuppuswamy, Sathyanarayanan wrote:
> 
> On 8/4/21 11:31 AM, Sean Christopherson wrote:
> > > On Intel CPUs writing the CSTAR MSR is not really needed. Syscalls
> > > from 32bit work using SYSENTER and 32bit SYSCALL is an illegal opcode.
> > > But the kernel did write it anyways even though it was ignored by
> > > the CPU. Inside a TDX guest this actually leads to a #GP. While the #GP
> > > is caught and recovered from, it prints an ugly message at boot.
> > > Do not write the CSTAR MSR on Intel CPUs.
> > Not that it really matters, but...
> > 
> > Is #GP the actual TDX-Module behavior?  If so, isn't that a contradiction with
> 
> No, #GP is triggered by guest.

#GP is not triggered by the guest, it's not even reported by the guest.  From
patch 7, the #VE handler escalates unhandled #VEs "similar to #GP handler", but
it still reports #VE as the actual vector.

Now, that particular behavior could change, e.g. setting tsk->thread.trap_nr to
#VE might confuse userspace, but at no point does this "trigger" a #GP.