linux-kernel - Re: [PATCH v10 10/11] x86/tdx: Don't write CSTAR MSR on Intel

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Date:   Thu, 14 Oct 2021 06:47:14 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Peter H Anvin <hpa@...or.com>, Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 10/11] x86/tdx: Don't write CSTAR MSR on Intel


>> -	wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
>> +	/*
>> +	 * CSTAR is not needed on Intel because it doesn't support
>> +	 * 32bit SYSCALL, but only SYSENTER. On a TDX guest
>> +	 * it leads to a #GP.
> Sigh. Above you write it raises #VE, but now it's #GP !?!


The unhandled #VE trap is handled like a #GP, which is then caught by 
the kernel wrmsr code.

So both are correct.

>
>          Intel CPUs do not support 32-bit SYSCALL. Writing to MSR_CSTAR
>          is normaly ignored by the CPU, but raises a #VE trap in a TDX
>          guest.
>
> Hmm?