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Subject: Re: [Xen-devel] [PATCH v2 1/2] x86: respect memory size limiting via
 mem= parameter
To:     Ingo Molnar <mingo@kernel.org>
Cc:     sstabellini@kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, xen-devel@lists.xenproject.org,
        boris.ostrovsky@oracle.com, tglx@linutronix.de
References: <20190130082233.23840-1-jgross@suse.com>
 <20190130082233.23840-2-jgross@suse.com> <20190211120650.GA74879@gmail.com>
From:   Juergen Gross <jgross@suse.com>
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Message-ID: <bd5863a2-291a-43e5-7633-c84c1026a31b@suse.com>
Date:   Mon, 11 Feb 2019 13:14:59 +0100
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On 11/02/2019 13:06, Ingo Molnar wrote:
> 
> * 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.

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