linux-kernel - Re: [PATCH v5 12/12] x86/tdx: Handle CPUID via #VE

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Date:   Tue, 24 Aug 2021 19:48:37 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 12/12] x86/tdx: Handle CPUID via #VE

On Wed, Aug 04, 2021 at 11:13:29AM -0700, Kuppuswamy Sathyanarayanan wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.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 hypercall to the VMM,
> or handled directly.
> 
> The TDX module EAS has a full list of CPUID leaves which are handled

EAS?

> natively or by the TDX module in 16.2. Only unknown CPUIDs are handled by

16.2?

I believe that commit message was forgotten to be converted to
outside-Intel speak. Please do so.

> the #VE method. In practice this typically only applies to the
> hypervisor specific CPUIDs unknown to the native CPU.

hypervisor-specific

> 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@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> ---
> 
> Changes since v4:
>  * None
> 
> Changes since v3:
>  * None
> 
>  arch/x86/kernel/tdx.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index d16c7f8759ea..5d2fd6c8b01c 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -153,6 +153,21 @@ static int tdg_write_msr_safe(unsigned int msr, unsigned int low,
>  	return ret ? -EIO : 0;
>  }
>  
> +static void tdg_handle_cpuid(struct pt_regs *regs)
> +{
> +	u64 ret;
> +	struct tdx_hypercall_output out = {0};
> +
> +	ret = _tdx_hypercall(EXIT_REASON_CPUID, regs->ax, regs->cx, 0, 0, &out);
> +
> +	WARN_ON(ret);

Why warn and not forward the error, instead, so that it lands in
ve_raise_fault() ?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette