On 04.06.20 17:27, David Hildenbrand wrote:

On 04.06.20 17:06, Christoph Hellwig wrote:

quoted

On Thu, Jun 04, 2020 at 01:32:40PM +0200, David Hildenbrand wrote:

quoted

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


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