Re: [PATCH v2 09/20] KVM: s390: KVM page table management functions: clear and replace

Janosch Frank
2025-09-11T12:57:48Z

On 9/10/25 8:07 PM, Claudio Imbrenda wrote:
> Add page table management functions to be used for KVM guest (gmap)
> page tables.
>
> This patch adds functions to clear, replace or exchange DAT table
> entries.
>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
> arch/s390/kvm/dat.c | 120 ++++++++++++++++++++++++++++++++++++++++++++
> arch/s390/kvm/dat.h | 40 +++++++++++++++
> 2 files changed, 160 insertions(+)
>
> diff --git a/arch/s390/kvm/dat.c b/arch/s390/kvm/dat.c
> index 326be78adcda..f26e3579bd77 100644
> --- a/arch/s390/kvm/dat.c
> +++ b/arch/s390/kvm/dat.c
> @@ -89,3 +89,123 @@ void dat_free_level(struct crst_table *table, bool owns_ptes)
> }
> dat_free_crst(table);
> }
> +
> +/**
> + * dat_crstep_xchg - exchange a guest CRST with another
> + * @crstep: pointer to the CRST entry
> + * @new: replacement entry
> + * @gfn: the affected guest address
> + * @asce: the ASCE of the address space
> + *
> + * This function is assumed to be called with the guest_table_lock
> + * held.
> + */
> +void dat_crstep_xchg(union crste *crstep, union crste new, gfn_t gfn, union asce asce)
> +{
> + if (crstep->h.i) {
> + WRITE_ONCE(*crstep, new);
> + return;
> + } else if (cpu_has_edat2()) {
> + crdte_crste(crstep, *crstep, new, gfn, asce);
> + return;
> + }
> +
> + if (machine_has_tlb_guest())
> + idte_crste(crstep, gfn, IDTE_GUEST_ASCE, asce, IDTE_GLOBAL);
> + else if (cpu_has_idte())
> + idte_crste(crstep, gfn, 0, NULL_ASCE, IDTE_GLOBAL);
> + else
> + csp_invalidate_crste(crstep);

I'm wondering if we can make stfle 3 (DTE) a requirement for KVM or
Linux as a whole since it was introduced with z990 AFAIK.

> + WRITE_ONCE(*crstep, new);
> +}
> +
> +/**
> + * dat_crstep_xchg_atomic - exchange a gmap pmd with another
> + * @crstep: pointer to the crste entry
> + * @old: expected old value
> + * @new: replacement entry
> + * @gfn: the affected guest address
> + * @asce: the asce of the address space
> + *
> + * This function should only be called on invalid crstes, or on crstes with
> + * FC = 1, as that guarantees the presence of CSPG.
> + *
> + * Return: true if the exchange was successful.
> + */
> +bool dat_crstep_xchg_atomic(union crste *crstep, union crste old, union crste new, gfn_t gfn,
> + union asce asce)
> +{
> + if (old.h.i)
> + return arch_try_cmpxchg((long *)crstep, &old.val, new.val);
> + if (cpu_has_edat2())
> + return crdte_crste(crstep, old, new, gfn, asce);
> + if (cpu_has_idte())
> + return cspg_crste(crstep, old, new);
> +
> + WARN_ONCE(1, "Machine does not have CSPG and DAT table was not invalid.");
> + return false;
> +}