On Sat, Jun 27, 2020 at 12:53 AM Kaneda, Erik <[email protected]> wrote: > > > > > -----Original Message----- > > From: Rafael J. Wysocki <[email protected]> > > Sent: Monday, June 22, 2020 7:02 AM > > To: Williams, Dan J <[email protected]>; Kaneda, Erik > > <[email protected]> > > Cc: Wysocki, Rafael J <[email protected]>; Len Brown > > <[email protected]>; Borislav Petkov <[email protected]>; Weiny, Ira > > <[email protected]>; James Morse <[email protected]>; Myron > > Stowe <[email protected]>; Andy Shevchenko > > <[email protected]>; [email protected]; linux- > > [email protected]; [email protected]; Moore, Robert > > <[email protected]> > > Subject: [RFT][PATCH v2 3/4] ACPICA: Preserve memory opregion mappings > > if supported by OS > > > > From: "Rafael J. Wysocki" <[email protected]> > > > > The ACPICA's strategy with respect to the handling of memory mappings > > associated with memory operation regions is to avoid mapping the > > entire region at once which may be problematic at least in principle > > (for example, it may lead to conflicts with overlapping mappings > > having different attributes created by drivers). It may also be > > wasteful, because memory opregions on some systems take up vast > > chunks of address space while the fields in those regions actually > > accessed by AML are sparsely distributed. > > > > For this reason, a one-page "window" is mapped for a given opregion > > on the first memory access through it and if that "window" does not > > cover an address range accessed through that opregion subsequently, > > it is unmapped and a new "window" is mapped to replace it. Next, > > if the new "window" is not sufficient to access memory through the > > opregion in question in the future, it will be replaced with yet > > another "window" and so on. That may lead to a suboptimal sequence > > of memory mapping and unmapping operations, for example if two fields > > in one opregion separated from each other by a sufficiently wide > > chunk of unused address space are accessed in an alternating pattern. > > > > The situation may still be suboptimal if the deferred unmapping > > introduced previously is supported by the OS layer. For instance, > > the alternating memory access pattern mentioned above may produce > > a relatively long list of mappings to release with substantial > > duplication among the entries in it, which could be avoided if > > acpi_ex_system_memory_space_handler() did not release the mapping > > used by it previously as soon as the current access was not covered > > by it. > > > > In order to improve that, modify acpi_ex_system_memory_space_handler() > > to take advantage of the memory mappings reference counting at the OS > > level if a suitable interface is provided. > > > Hi, > > > Namely, if ACPI_USE_FAST_PATH_MAPPING is set, the OS is expected to > > implement acpi_os_map_memory_fast_path() that will return NULL if > > there is no mapping covering the given address range known to it. > > If such a mapping is there, however, its reference counter will be > > incremented and a pointer representing the requested virtual address > > will be returned right away without any additional consequences. > > I do not fully understand why this is under a #ifdef. Is this to support > operating systems that might not want to add support for this behavior?
Yes, and to protect the ones that have not added support for it just yet. Without the "fast-path" mapping support, ACPICA has no way to obtain additional references to known-existing mappings and the new code won't work as expected without it, so it is better to avoid building that code at all in those cases IMO. > Also, instead of using the terminology fast_path, I think it would be easier > to use terminology that describes the mechanism.. > It might be easier for other Operating systems to understand something like > acpi_os_map_preserved_memory or acpi_os_map_sysmem_opregion_memory. Well, the naming is not particularly important to me to be honest, but this is mostly about being able to get a new reference to a known-existing memory mapping. So something like acpi_os_ref_memory_map() perhaps? But I'm thinking that this can be implemented without the "fast-path" mapping support too, let me try to do that. Cheers!
