On December 4, 2024 5:29:17 AM PST, Brian Gerst <brgerst@xxxxxxxxx> wrote: >On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: >> >> From: Arnd Bergmann <arnd@xxxxxxxx> >> >> The HIGHMEM64G support was added in linux-2.3.25 to support (then) >> high-end Pentium Pro and Pentium III Xeon servers with more than 4GB of >> addressing, NUMA and PCI-X slots started appearing. >> >> I have found no evidence of this ever being used in regular dual-socket >> servers or consumer devices, all the users seem obsolete these days, >> even by i386 standards: >> >> - Support for NUMA servers (NUMA-Q, IBM x440, unisys) was already >> removed ten years ago. >> >> - 4+ socket non-NUMA servers based on Intel 450GX/450NX, HP F8 and >> ServerWorks ServerSet/GrandChampion could theoretically still work >> with 8GB, but these were exceptionally rare even 20 years ago and >> would have usually been equipped with than the maximum amount of >> RAM. >> >> - Some SKUs of the Celeron D from 2004 had 64-bit mode fused off but >> could still work in a Socket 775 mainboard designed for the later >> Core 2 Duo and 8GB. Apparently most BIOSes at the time only allowed >> 64-bit CPUs. >> >> - In the early days of x86-64 hardware, there was sometimes the need >> to run a 32-bit kernel to work around bugs in the hardware drivers, >> or in the syscall emulation for 32-bit userspace. This likely still >> works but there should never be a need for this any more. >> >> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOTLB. >> PAE mode is still required to get access to the 'NX' bit on Atom >> 'Pentium M' and 'Core Duo' CPUs. > >8GB of memory is still useful for 32-bit guest VMs. > > >Brian Gerst > By the way, there are 64-bit machines which require swiotlb.
