linux-kernel - Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software defined MTRRs

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Date:   Mon, 13 Feb 2023 16:03:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        lists@...dbynature.de, mikelley@...rosoft.com,
        torvalds@...ux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs

On Mon, Feb 13, 2023 at 03:07:07PM +0100, Juergen Gross wrote:
> Fixed in the sense of static.

Well, you can't use "fixed" to say "static" when former means something
very specific already in MTRR land.

> Wouldn't !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) be enough?
> 
> I'm not sure we won't need that for TDX guests, too.

See, that's the problem. I wanna have it simple too. Lemme check with
dhansen.

> Yes, it is only relevant for PV dom0.

Right, I was asking whether "PV dom0" == X86_FEATURE_XENPV?

:)

> The number of fixed MTRRs is not dynamic AFAIK.

But nothing guarantees that the caller would pass an array "mtrr_type
*fixed" of size MTRR_NUM_FIXED_RANGES, right?

> A single interface makes it easier to avoid multiple calls.
> 
> In the end I'm fine with either way.

Yeah, I know. Question is, how much of this functionality will be
needed/used so that we can go all out on the interface design or we can
do a single one and forget about it...

> > Can Xen use x86_hyper_type() too?
> 
> It does.

Then pls add a x86_hyper_type check too to make sure a potential move of
this call is caught in the future.

Thx.

-- 
Regards/Gruss,
    Boris.

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