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

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Date:   Tue, 14 Feb 2023 10:17:12 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
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 14.02.23 10:10, Borislav Petkov wrote:
> On Tue, Feb 14, 2023 at 10:02:51AM +0100, Juergen Gross wrote:
>> I just don't like the idea of trying to catch all possible misuses in
>> lower levels, at the same time introducing the need to modify those
>> tests in case a new valid use case is popping up.
> 
> So what would you do: generally allow this so that potentially other
> guest configurations misuse it?

I guess this largely depends on the functionality. I don't see why anyone
would try to use MTRR overwrite functionality without really needing it.

But maybe I'm wrong here and I'm under-estimating the "creativity" of
kernel hackers.

> And when we decide to change it, all those users will come running and
> complaining that we broke it?
> 
> And then we're stuck with a nasty workaround in the tree because we have
> to support them too?
> 
> See, all we do here is because of such misguided (or maybe didn't know
> better) decisions which have happened a long time ago.

I can see your point.

Maybe I haven't seen enough crazy hacks yet. :-)

No need to further discuss this topic from my side, as I have voiced my
opinion and you did so, too. I will add the tests you are asking for.


Juergen

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