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

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Date:   Fri, 28 Oct 2022 16:27:10 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     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()

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.

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.