linux-kernel - Re: [PATCH v7 RESEND 4/4] kdump/vmcore: support encrypted old memory with SME enabled

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Message-ID: <20180929083052.GA13786@zn.tnic>
Date:   Sat, 29 Sep 2018 10:30:52 +0200
From:   Borislav Petkov <bp@...e.de>
To:     lijiang <lijiang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, thomas.lendacky@....com,
        bhelgaas@...gle.com, baiyaowei@...s.chinamobile.com, tiwai@...e.de,
        brijesh.singh@....com, dyoung@...hat.com, bhe@...hat.com,
        jroedel@...e.de
Subject: Re: [PATCH v7 RESEND 4/4] kdump/vmcore: support encrypted old memory
 with SME enabled

On Sat, Sep 29, 2018 at 02:24:52PM +0800, lijiang wrote:
> At first, i added an input parameter for read_from_oldmem() because of encryption(SME). But
> for avoiding to also add the same parameter for copy_oldmem_page(), so i added a new function
> copy_oldmem_page_encrypted(). Maybe you had noticed that these functions were very similar.

If you have two very similar functions, you add a *static* workhorse function:

static ssize_t __copy_oldmem_page(unsigned long pfn, char *buf, size_t csize, unsigned long offset,
				  int userbuf, bool encrypted)

and you define two wrappers:

copy_oldmem_page()
copy_oldmem_page_encrypted()

which both call __copy_oldmem_page() with the appropriate parameters.

But you do *not* do a separate compilation unit just because. None of
the reasons you've mentioned warrant having a separate compilation
unit while you already have *the* perfect place to put everything -
arch/x86/kernel/crash_dump_64.c

> So sorry, here "oldmem encrypted" means the "old memory is encrypted".

I know what it means - I'm trying to explain to you to write it out
in plain english and not use some strange constructs like "oldmem
encrypted".

A reader would wonder: why is this thing semi-abbreviated and in
quotation marks? Does that mean anything special?

Our comments should not be write-only. So after you've written it, try
to read it as someone who sees the code for the first time and think
hard whether she/he will understand it.

Do you catch my drift now?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)