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Subject: Re: [PATCH] iommu/mediatek: Use totalram_pages to setup enable_4GB
From:   David Hildenbrand <david@redhat.com>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Miles Chen <miles.chen@mediatek.com>, wsd_upstream@mediatek.com,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org,
        Chao Hao <chao.hao@mediatek.com>,
        iommu@lists.linux-foundation.org,
        linux-mediatek@lists.infradead.org, Yong Wu <yong.wu@mediatek.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        yingjoe.chen@mediatek.com, linux-arm-kernel@lists.infradead.org
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Date:   Fri, 5 Jun 2020 14:52:32 +0200
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On 04.06.20 17:27, David Hildenbrand wrote:
> On 04.06.20 17:06, Christoph Hellwig wrote:
>> On Thu, Jun 04, 2020 at 01:32:40PM +0200, David Hildenbrand wrote:
>>> Just a thought: If memory hotplug is applicable as well, you might
>>> either want to always assume data->enable_4GB, or handle memory hotplug
>>> events from the memory notifier, when new memory gets onlined (not sure
>>> how tricky that is).
>>
>> We probably want a highest_pfn_possible() or similar API instead of
>> having drivers poking into random VM internals.
> 
> Well, memory notifiers are a reasonable api used accross the kernel to
> get notified when new memory is onlined to the buddy that could be used
> for allocations.
> 
> highest_pfn_possible() would have to default to something linked to
> MAX_PHYSMEM_BITS whenever memory hotplug is configured, I am not sure
> how helpful that is (IOW, you can just default to enable_4GB=true in
> that case instead in most cases).

Correction: At least on x86-64 we have max_possible_pfn, which will
consult the ACPI SRAT table to figure out the maximum possible PFN.
(Without SRAT, max_possible_pfn will point at the end of initial boot
memory and not consider hotplug memory - something that e.g., newer QEMU
versions work around by creating SRAT tables if memory hotplug might be
possible, even if there is no actual NUMA configuration).

pci-swiotlb.c similarly relies on that to figure out if there are any
!DMA addresses to handle.

-- 
Thanks,

David / dhildenb