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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 5 Feb 2021 15:42:01 -0800
Message-ID: <CALCETrW3+mzHaCUcWC1YUSwiuHkDWo=S4xnavk0CMcBiZd17+Q@mail.gmail.com>
Subject: Re: [RFC v1 09/26] x86/tdx: Handle CPUID via #VE
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Feb 5, 2021 at 3:39 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> TDX has three classes of CPUID leaves: some CPUID leaves
> are always handled by the CPU, others are handled by the TDX module,
> and some others are handled by the VMM. Since the VMM cannot directly
> intercept the instruction these are reflected with a #VE exception
> to the guest, which then converts it into a TDCALL to the VMM,
> or handled directly.
>
> The TDX module EAS has a full list of CPUID leaves which are handled
> natively or by the TDX module in 16.2. Only unknown CPUIDs are handled by
> the #VE method. In practice this typically only applies to the
> hypervisor specific CPUIDs unknown to the native CPU.
>
> Therefore there is no risk of causing this in early CPUID code which
> runs before the #VE handler is set up because it will never access
> those exotic CPUID leaves.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> ---
>  arch/x86/kernel/tdx.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index 5d961263601e..e98058c048b5 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -172,6 +172,35 @@ static int tdx_write_msr_safe(unsigned int msr, unsigned int low,
>         return ret || r10 ? -EIO : 0;
>  }
>
> +static void tdx_handle_cpuid(struct pt_regs *regs)
> +{
> +       register long r10 asm("r10") = TDVMCALL_STANDARD;
> +       register long r11 asm("r11") = EXIT_REASON_CPUID;
> +       register long r12 asm("r12") = regs->ax;
> +       register long r13 asm("r13") = regs->cx;
> +       register long r14 asm("r14");
> +       register long r15 asm("r15");
> +       register long rcx asm("rcx");
> +       long ret;
> +
> +       /* Allow to pass R10, R11, R12, R13, R14 and R15 down to the VMM */
> +       rcx = BIT(10) | BIT(11) | BIT(12) | BIT(13) | BIT(14) | BIT(15);
> +
> +       asm volatile(TDCALL
> +                       : "=a"(ret), "=r"(r10), "=r"(r11), "=r"(r12), "=r"(r13),
> +                         "=r"(r14), "=r"(r15)
> +                       : "a"(TDVMCALL), "r"(rcx), "r"(r10), "r"(r11), "r"(r12),
> +                         "r"(r13)
> +                       : );

Some "+" constraints would make this simpler.  But I think you should
factor the TDCALL helper out into its own function.