On Tue, Apr 9, 2019 at 5:13 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 04, 2019 at 12:08:49PM -0700, Dan Williams wrote: > > Memory that has been tagged EFI_SPECIAL_PURPOSE, and has performance > > properties described by the ACPI HMAT is expected to have an application > > specific consumer. > > > > Those consumers may want 100% of the memory capacity to be reserved from > > any usage by the kernel. By default, with this enabling, a platform > > device is created to represent this differentiated resource. > > This sounds more than weird. Since when did we let the firmware decide > who can use the memory? There's 2 related motivations for playing along with this "special purpose" attribute. Before this bit we've seen gross hacks in platform firmware trying to game OS behavior by lying about numa distances in the ACPI SLIT. For example "near" high bandwidth memory being set at a large distance to prevent the kernel from allocating from it by default as much as possible. Secondly, allow niche applications guarantees about being able to claim all of a given designated resource. The above comes with the option to override this default reservation and just turn it back over to the page allocator i.e. ignore the platform firmware hint. The alternative is arranging for "special purpose" memory to be given to the page allocator by default with the hope that it can be reserved / claimed early so the administrator can prevent unwanted allocations. It just seemed overall easier to "default reserve with the option to hot-add" instead of "default online with option / hope of hot-remove or early allocation".
