linux-kernel - Re: [PATCH] x86/cpu/amd: Remove dead code for TSEG region remapping

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Date:   Fri, 27 Nov 2020 18:27:47 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86@...nel.org, Kim Phillips <kim.phillips@....com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pu Wen <puwen@...on.cn>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cpu/amd: Remove dead code for TSEG region remapping

On Fri, Nov 27, 2020 at 12:13:24PM -0500, Arvind Sankar wrote:
> Commit
>   26bfa5f89486 ("x86, amd: Cleanup init_amd")
> moved the code that remaps the TSEG region using 4k pages from
> init_amd() to bsp_init_amd().
> 
> However, bsp_init_amd() is executed well before the direct mapping is
> actually created:
> 
>   setup_arch()
>     -> early_cpu_init()
>       -> early_identify_cpu()
>         -> this_cpu->c_bsp_init()
> 	  -> bsp_init_amd()
>     ...
>     -> init_mem_mapping()
> 
> So the change effectively disabled the 4k remapping, because
> pfn_range_is_mapped() is always false at this point.
> 
> It has been over six years since the commit, and no-one seems to have
> noticed this, so just remove the code. The original code was also
> incomplete, since it doesn't check how large the TSEG address range
> actually is, so it might remap only part of it in any case.

Yah, and the patch which added this:

6c62aa4a3c12 ("x86: make amd.c have 64bit support code")

does not say what for (I'm not surprised, frankly).

So if AMD folks on Cc don't have any need for actually fixing this
properly, yap, we can zap it.

Thx.

-- 
Regards/Gruss,
    Boris.

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