linux-kernel - Re: [PATCH 19/26] x86/tdx: Make pages shared in ioremap()

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Message-ID: <20220105094611.kugiarylwjjjttkc@box.shutemov.name>
Date:   Wed, 5 Jan 2022 12:46:11 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Borislav Petkov <bp@...en8.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>, tglx@...utronix.de,
        mingo@...hat.com, luto@...nel.org, peterz@...radead.org,
        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,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 19/26] x86/tdx: Make pages shared in ioremap()

On Tue, Jan 04, 2022 at 05:38:25PM -0800, Dave Hansen wrote:
> On 1/4/22 4:57 PM, Kirill A. Shutemov wrote:
> >> My read of STRICT_MM_TYPECHECKS was that "typedef unsigned long
> >> pgprot_t" produces better code, but "typedef struct { unsigned long
> >> pgprot; } pgprot_t;" produces better type checking.
> > Apart from pgprot_t, __pgprot() and pgrot_val() helpers are defined
> > differently depending on STRICT_MM_TYPECHECKS.
> > 
> >> I just compiled these patches on sparc with no issues.
> > Hm. I can't see how
> > 
> > #define pgprot_val(x)	(x)
> > 
> > can work to access value for the pgprot_t defined as a struct.
> 
> Oh, I must just be compiling with the strict type checks on all the
> time.  I do really wonder if these are useful these days or if the hacks
> were for ancient compilers.
> 
> In any case, this would be pretty easy to fix by just removing the
> !STRICT_MM_TYPECHECKS pgprot_val() and defning the STRICT_MM_TYPECHECKS
> universally.

There's comment in 32-bit Sparc as a reason for STRICT_MM_TYPECHECKS not
to be used:

/* passing structs on the Sparc slow us down tremendously... */

The comment came from before git times, so I don't know if it still has a
merit or newer compilers can deal with it better.

bloat-o-meter shows not trivial difference:

Total: Before=5342261, After=5344025, chg +0.03%

but I'm not sure if it translates into performance loss.

David, does the comment still relevant?

-- 
 Kirill A. Shutemov