Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed XENMEM_exchange requests

[Christoph Hellwig]

On Tue, Sep 07, 2021 at 02:05:32PM +0200, Jan Beulich wrote:
> While the hypervisor hasn't been enforcing this, we would still better
> avoid issuing requests with GFNs not aligned to the requested order.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> I wonder how useful it is to include the alignment in the panic()
> message. I further wonder how useful it is to wrap "bytes" in
> PAGE_ALIGN(), when it is a multiple of a segment's size anyway (or at
> least was supposed to be, prior to "swiotlb-xen: maintain slab count
> properly").
> 
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -231,10 +231,10 @@ retry:
>  	/*
>  	 * Get IO TLB memory from any location.
>  	 */
> -	start = memblock_alloc(PAGE_ALIGN(bytes), PAGE_SIZE);
> +	start = memblock_alloc(PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);
>  	if (!start)
> -		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
> -		      __func__, PAGE_ALIGN(bytes), PAGE_SIZE);
> +		panic("%s: Failed to allocate %lu bytes align=%#x\n",
> +		      __func__, PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);

CAn you avoid the overly long lines here?  A good way to make it more
readable would be a variable to hold the byte count.