Re: [PATCH v2 4/4] KVM: s390: Replace get_zeroed_page() with kzalloc() for sie_page2 and CMMA

[Janosch Frank <frankja@linux.ibm.com>]

On 9/6/26 10:27 AM, Mike Rapoport (Microsoft) wrote:
> kvm_arch_init_vm() allocates struct sie_page2 that holds the facility
> list and the crypto control block of a VM and kvm_s390_vcpu_setup_cmma()
> allocates the CMMA collaborative memory management block referenced by
> the SIE control block. Both are passed to the hardware as physical
> addresses and must be page aligned.
> 
> kmalloc() guarantees that a power of two sized allocation is aligned to
> its size, so a PAGE_SIZE allocation is page aligned as well.
> 
> These buffers can be allocated with kmalloc() as there's nothing special
> about them to go directly to the page allocator.
> 
> kmalloc() provides a better API that does not require ugly casts and
> kfree() does not need to know the size of the freed object.
> 
> Performance difference between kmalloc() and __get_free_pages() is not
> measurable as both allocators take an object/page from a per-CPU list for
> fast path allocations.
> 
> For the slow path the performance is anyway determined by the amount of
> reclaim involved rather than by what allocator is used.
> 
> The sie_page allocated in kvm_arch_vcpu_create() is left alone because it
> is mapped to userspace with virt_to_page() in kvm_arch_vcpu_fault() and
> therefore really needs a struct page.
> 
> The SIE hardware stores the offset of the next CBR entry in the low bits
> of cbrlo, so mask them out before converting cbrlo back to a virtual
> address. free_page() did not care, kfree() needs the address kzalloc()
> returned.
> 
> Replace use of get_zeroed_page() with kzalloc() and free_page() with
> kfree().
> 
> Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
> Assisted-by: copilot:claude-opus
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>