Re: [PATCH 05/11] x86: remove HIGHMEM64G support

[Brian Gerst]

On Wed, Dec 4, 2024 at 8:43 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Dec 4, 2024, at 14:29, Brian Gerst wrote:
> > On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> >>
> >>  - 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.
>
> Can you give some more background on this?
>
> It's clear that one can run a virtual machine this way and it
> currently works, but are you able to construct a case where this
> is a good idea, compared to running the same userspace with a
> 64-bit kernel?
>
> From what I can tell, any practical workload that requires
> 8GB of total RAM will likely run into either the lowmem
> limits or into virtual addressig limits, in addition to the
> problems of 32-bit kernels being generally worse than 64-bit
> ones in terms of performance, features and testing.

I use a 32-bit VM to test 32-bit kernel builds.  I haven't benchmarked
kernel builds with 4GB/8GB yet, but logically more memory would be
better for caching files.


Brian Gerst