linux-kernel - Re: [Xen-devel] [PATCH v2 1/2] x86: respect memory size limiting via mem= parameter

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Date:   Mon, 11 Feb 2019 13:14:59 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     sstabellini@...nel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, xen-devel@...ts.xenproject.org,
        boris.ostrovsky@...cle.com, tglx@...utronix.de
Subject: Re: [Xen-devel] [PATCH v2 1/2] x86: respect memory size limiting via
 mem= parameter

On 11/02/2019 13:06, Ingo Molnar wrote:
> 
> * Juergen Gross <jgross@...e.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.

Right.

> 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.

No, in this case it seems to be real RAM added via PCI. The RAM is
initially present in the E820 map, but the "mem=" will remove it from
there again. During ACPI scan it is found (again) and will be added
via hotplug mechanism, so "mem=" has no effect for that memory.


Juergen