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Date:   Mon, 11 Feb 2019 13:06:50 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org, linux-mm@kvack.org, boris.ostrovsky@oracle.com,
        sstabellini@kernel.org, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de
Subject: Re: [PATCH v2 1/2] x86: respect memory size limiting via mem=
 parameter
Message-ID: <20190211120650.GA74879@gmail.com>
References: <20190130082233.23840-1-jgross@suse.com>
 <20190130082233.23840-2-jgross@suse.com>
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* Juergen Gross <jgross@suse.com> wrote:

> When limiting memory size via kernel parameter "mem=" this should be
> respected even in case of memory made accessible via a PCI card.
> 
> Today this kind of memory won't be made usable in initial memory
> setup as the memory won't be visible in E820 map, but it might be
> added when adding PCI devices due to corresponding ACPI table entries.
> 
> Not respecting "mem=" can be corrected by adding a global max_mem_size
> variable set by parse_memopt() which will result in rejecting adding
> memory areas resulting in a memory size above the allowed limit.

So historically 'mem=xxxM' was a way to quickly limit RAM.

If PCI devices had physical mmio memory areas above this range, we'd 
still expect them to work - the option was really only meant to limit 
RAM.

So I'm wondering what the new logic is here - why should an iomem 
resource from a PCI device be ignored? It's a completely separate area 
that might or might not be enumerated in the e820 table - the only 
requirement we have here I think is that it not overlap RAM areas or each 
other (obviously).

So if I understood this new restriction you want mem= to imply, devices 
would start failing to initialize on bare metal when mem= is used?

Thanks,

	Ingo