Re: [PATCH v2 1/4] KVM: s390: Replace get_zeroed_page() with kzalloc() for the STHYI buffer

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

On 9/6/26 10:27 AM, Mike Rapoport (Microsoft) wrote:
> handle_sthyi() allocates the buffer that receives the STHYI response
> block before it is copied to the guest.
> 
> This buffer can be allocated with kmalloc() as there's nothing special
> about it 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 only goto in that function does not unwind anything, it merely skips
> to the exit when the function code is invalid. Turn it into an early
> return so that the buffer can be freed with __free(kfree).
> 
> Replace use of get_zeroed_page() with kzalloc() and free_page() with
> kfree().
> 
> Assisted-by: copilot:claude-opus
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Yup, there's no alignment requirement anyway.
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>



Looking at the kernel's sthyi.c there are a couple more locations we 
could change:

I don't see a need for the syscall to allocate a page and the cache 
doesn't need page allocation either. We should even be able to have them 
byte aligned since they are never the destination for one of the 
diagnose instructions, a kvzalloc should be fine if I didn't miss 
something...


Diag224 needs a 4k aligned 31-bit physical address for some reason so 
that needs GFP_DMA. diag204 is vmallocing anyway since subcode 7 uses 
logical addresses.