linux-kernel - Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc

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Date:   Sat, 29 Oct 2022 02:59:51 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, ak@...ux.intel.com,
        dan.j.williams@...el.com, david@...hat.com, hpa@...or.com,
        seanjc@...gle.com, thomas.lendacky@....com,
        elena.reshetova@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc_mask()

On Fri, Oct 28, 2022 at 04:27:10PM -0700, Dave Hansen wrote:
> I looked at this a bit more closely.  The code is just bonkers now.  It
> can't go in like this.
> 
> tdx_parse_tdinfo() stashes two global variables.  Then, about three
> lines later in the function, it calls get_cc_mask() which just returns
> one of those variables, modulo a little masking.
> 
> Ditto for the td_attr.  It's stashed in a global variable and then read
> one time.
> 
> There is *ZERO* reason to store 'td_attr'.  There's also zero reason to
> have 'gpa_width' as a global variable.  It's only used *ONE* *TIME* from
> the scope of *ONE* *FUNCTION*.
> 
> Even the comment is bonkers:
> 
>         /*
>          * Initializes gpa_width and td_attr. Must be called before
>          * get_cc_mask() or attribute checks.
>          */
>         tdx_parse_tdinfo();
> 
> Comments are great.  But comments that are only there because the code
> is obtuse are not.  I changed it to:
> 
>         tdx_parse_tdinfo(&cc_mask);
> 
> It doesn't even need a comment now.  Why?  Because it's obvious from the
> naming and calling convention that it initializes cc_mask and what the
> ordering dependency is.  Plus, even *if* I missed the call, or screwed
> up the order, the compiler would tell me that I'm a dolt.
> 
> The whole global variable thing actually makes the code objectively
> worse in almost every possible way.

I agree. Sorry about that.

We have more code in our tree that want to check attributes. And it is
after initialization, so the code structure derived from there.

But, yes, I should have rework it before sending upstream.

> Can you please take a look through this and make sure I didn't botch
> anything:
> 
> > https://git.kernel.org/pub/scm/linux/kernel/git/daveh/devel.git/log/?h=tdxbadve
> 
> The end result is about 50 lines less than what was there before.  Most
> of it is comment removal but the code is simpler too.
> 
> Acks and Tested-by's would be appreciated.

Looks good and works fine:

Acked-and-Tested-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov