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Date:   Tue, 19 Jul 2022 17:12:38 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Bharata B Rao <bharata@amd.com>
Cc:     Borislav Petkov <bp@alien8.de>, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
        dave.hansen@linux.intel.com, nikunj@amd.com, hpa@zytor.com,
        Abraham.Shaju@amd.com
Subject: Re: [RFC FIX PATCH] x86/e820: Stop kernel boot when RAM resource
 reservation fails
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References: <20220718085815.1943-1-bharata@amd.com>
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On Tue, Jul 19, 2022, Bharata B Rao wrote:
> On 7/18/2022 8:37 PM, Borislav Petkov wrote:
> > 
> > I betcha you can generate a lot of "kernel bugs" with weird qemu
> > options. If it is not a real use case, nobody cares.
> 
> I see that we will hit this problem by default when starting
> a guest with 1T or more memory using QEMU.

That a user can create a bad configuration using QEMU's default MAXPHYADDR doesn't
change the fact that adding memory beyond MAXPHYADDR is firmly a configuration bug.

> > And even if it were a real use case, panicking the machine is not the
> > right fix.
> 
> I couldn't see a clean exit/recovery option in setup_arch()->e820__reserve_resources()
> where this happens. Any suggestions?

WARN or pr_err/warn() and move on, or just do nothing.  Adding code to try and
gracefully handle an architecturally impossible configuration is a waste of time
and effort.  Like Boris said, there's practically a limitless number of bad setups
QEMU can create, this one just happens to be easier to create than others due to
shortcomings in QEMU.