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

[Marc Zyngier]

On Wed, 30 Oct 2024 10:18:11 +0000,
Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> 
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.