linux-kernel - Re: [PATCHv6 04/30] x86/tdx: Extend the confidential computing API to support TDX guests

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Date:   Thu, 17 Mar 2022 01:01:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCHv6 04/30] x86/tdx: Extend the confidential computing API
 to support TDX guests

On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> +
> +static void get_info(unsigned int *gpa_width)

get_info() is really not a descriptive function name. Also why does this
need a pointer and does not return gpa_width?

>  
>  	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
>  
> +	get_info(&gpa_width);
> +
> +	cc_set_vendor(CC_VENDOR_INTEL);
> +
> +	/*
> +	 * The highest bit of a guest physical address is the "sharing" bit.
> +	 * Set it for shared pages and clear it for private pages.
> +	 */
> +	cc_set_mask(BIT_ULL(gpa_width - 1));

The code ordering here is silly:

    1) Retrieve gpa width
    2) Set CC vendor
    3) Use gpa width

static u64 get_cc_mask(void)
{
        gpa_width = magic();

        /*
         * Comment explaining the return value
         */
        return BIT_ULL(gpa_width - 1);
}

and

        cc_set_vendor(CC_VENDOR_INTEL);
        cc_set_mask(get_cc_mask());

would be too intuitive, right?

Thanks,

        tglx

---

“My tastes are simple: I am easily satisfied with the best.”
                                     ― Winston S. Churchill