Re: [kvm-unit-tests PATCH v1 3/4] s390x: mmu: add support for large pages

[Thomas Huth <thuth@redhat.com>]

On 09/02/2021 15.38, Claudio Imbrenda wrote:
> Add support for 1M and 2G pages.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
>   lib/s390x/mmu.h |  73 +++++++++++++-
>   lib/s390x/mmu.c | 246 +++++++++++++++++++++++++++++++++++++++++++-----
>   2 files changed, 294 insertions(+), 25 deletions(-)
[...]
> +/*
> + * Get the pte (page) DAT table entry for the given address and pmd,
> + * allocating it if necessary.
> + * The pmd must not be large.
> + */
> +static inline pte_t *get_pte(pmd_t *pmd, uintptr_t vaddr)
> +{
>   	pte_t *pte = pte_alloc(pmd, vaddr);
>   
> -	return &pte_val(*pte);
> +	assert(!pmd_large(*pmd));
> +	pte = pte_alloc(pmd, vaddr);

Why is this function doing "pte = pte_alloc(pmd, vaddr)" twice now?

> +	return pte;
> +}
[...]
> +	if ((level == 1) && !pgd_none(*(pgd_t *)ptr))
> +		idte_pgdp(va, ptr);
> +	else if ((level == 2) && !p4d_none(*(p4d_t *)ptr))
> +		idte_p4dp(va, ptr);
> +	else if ((level == 3) && !pud_none(*(pud_t *)ptr))
> +		idte_pudp(va, ptr);
> +	else if ((level == 4) && !pmd_none(*(pmd_t *)ptr))
> +		idte_pmdp(va, ptr);
> +	else if (!pte_none(*(pte_t *)ptr))
> +		ipte(va, ptr);

Meta-comment: Being someone who worked quite a bit with the page tables on 
s390x, but never really got in touch with the way it is handled in the Linux 
kernel, I'm always having a hard time to match all these TLAs to the PoP: 
pmd, pud, p4d ...
Can we please have a proper place in the kvm-unit-tests sources somewhere 
(maybe at the beginning of mmu.c), where the TLAs are explained and how they 
map to the region and segment tables of the Z architecture?
(I personally would prefer to completely switch to the Z arch naming 
instead, but I guess that's too much of a change right now)

  Thomas