linux-kernel - Re: [PATCH v6 00/16] x86/mtrr: fix handling with PAT but without MTRR

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Date:   Thu, 11 May 2023 18:32:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-hyperv@...r.kernel.org, linux-doc@...r.kernel.org,
        mikelley@...rosoft.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, Jonathan Corbet <corbet@....net>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 00/16] x86/mtrr: fix handling with PAT but without MTRR

On Wed, May 10, 2023 at 05:53:15PM +0200, Juergen Gross wrote:
> Urgh, yes, there is something missing:
> 
> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> index 031f7ea8e72b..9544e7d13bb3 100644
> --- a/arch/x86/kernel/cpu/mtrr/generic.c
> +++ b/arch/x86/kernel/cpu/mtrr/generic.c
> @@ -521,8 +521,12 @@ u8 mtrr_type_lookup(u64 start, u64 end, u8 *uniform)
>         for (i = 0; i < cache_map_n && start < end; i++) {
>                 if (start >= cache_map[i].end)
>                         continue;

So the loop will go through the map until...

> -               if (start < cache_map[i].start)
> +               if (start < cache_map[i].start) {

... it reaches the first entry where that is true.

>                         type = type_merge(type, mtrr_state.def_type, uniform);

the @type argument is MTRR_TYPE_INVALID, def_type is WRBACK so what
this'll do is simply get you the default WRBACK type:

type_merge:
        if (type == MTRR_TYPE_INVALID)
                return new_type;

> +                       start = cache_map[i].start;
> +                       if (end <= start)
> +                               break;

Now you break here because end <= start. Why?

You can just as well do:

	if (start < cache_map[i].start) {
		/* region non-overlapping with the region in the map */
		if (end <= cache_map[i].start)
			return type_merge(type, mtrr_state.def_type, uniform);

		... rest of the processing ...

In general, I get it that your code is slick but I want it to be
maintainable - not slick. I'd like for when people look at this, not
have to  add a bunch of debugging output in order to swap the whole
thing back into their brains.

So mtrr_type_lookup() definitely needs comments explaining what goes
where.

You can send it as a diff ontop - I'll merge it.

Thx.

-- 
Regards/Gruss,
    Boris.

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