[RFC PATCH 7/8] arm64/mm: Use reduced VA sizes (36/39/42 bits) only for user space

[Ard Biesheuvel]

On Wed, 30 Oct 2024 at 13:44, Marc Zyngier <maz at kernel.org> wrote:
>
> On Wed, 30 Oct 2024 10:18:11 +0000,
> Ard Biesheuvel <ardb+git at google.com> wrote:
> >
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > The advantage of a reduced virtual address space size is its impact on
> > the number of translation levels, which affects TLB pressure. The
> > working set of translations covering the kernel side is negligible
> > compared to user space, where each process has its own set of page
> > tables, and so most of the same benefit can be obtained by reducing the
> > VA size only for user space.
> >
> > As a preparatory step towards implementing this, drop all the reduced VA
> > space sizes in Kconfig, and replace it with a configurable userland VA
> > space size that is reflected in TASK_SIZE. This will be taken advantage
> > of in a subsequent patch to actually reduce the number of translations
> > used by the MMU for translating user space virtual addresses.
>
> I think this may have an impact on KVM's walking of the userspace page
> tables to determine whether we are trying to install a block mapping,
> which assumes that the start level and the number of VA bits are the
> same as the kernel (see get_user_mapping_size()).
>
> Probably nothing too complicated, but something to look into.
>

With the crude hack I used, things should just work, unless KVM reads
back the value of TTBR0_EL1.

But once that bit is implemented properly, it is definitely something
to be aware of. Thanks.