Re: [RFC net-next] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Pavel Begunkov]

On 7/28/25 19:46, Pavel Begunkov wrote:
> On 7/28/25 18:44, Mina Almasry wrote:
> > On Sun, Jul 27, 2025 at 9:21 PM Byungchul Park <byungchul@xxxxxx> wrote:
> > >
>
> ...>> - * Return: the netmem_ref cast to net_iov* regardless of its underlying type.
> > > + * Return: the pointer to struct netmem_desc * regardless of its
> > > + * underlying type.
> > >    */
> > > -static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> > > +static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
> > >   {
> > > -       return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> > > +       if (netmem_is_net_iov(netmem))
> > > +               return &((struct net_iov *)((__force unsigned long)netmem &
> > > +                                           ~NET_IOV))->desc;
> > > +
> > > +       return __netmem_to_nmdesc(netmem);
> >
> > The if statement generates overhead. I'd rather avoid it. We can
> > implement netmem_to_nmdesc like this, no?
> >
> > netmem_to_nmdesc(netmem_ref netmem)
> > {
> >    return (struct netmem_desc)((__force unsigned long)netmem & ~NET_IOV);
> > }
> >
> > Because netmem_desc is the first element in both net_iov and page for
> > the moment. (yes I know that will change eventually, but we don't have
> > to incur overhead of an extra if statement until netmem_desc is
> > removed from page, right?)
>
> Same concern, but I think the goal here should be to make enough

s/make/give/


> info to the compiler to optimise it out without assumptions on
> the layouts nor NET_IOV_ASSERT_OFFSET. Currently it's not so bad,
> but we should be able to remove this test+cmove.
>
>      movq    %rdi, %rax    # netmem, tmp105
>      andq    $-2, %rax    #, tmp105
>      testb    $1, %dil    #, netmem
>      cmove    %rdi, %rax    # tmp105,, netmem, <retval>
>      jmp    __x86_return_thunk

struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
{
	void *p = (void *)((__force unsigned long)netmem & ~NET_IOV);

	if (netmem_is_net_iov(netmem))
		return &((struct net_iov *)p)->desc;
	return __pp_page_to_nmdesc((struct page *)p);
}

movq	%rdi, %rax	# netmem, netmem
andq	$-2, %rax	#, netmem
jmp	__x86_return_thunk


This should do it, and if the layouts change, it'd still
remain correct.

--
Pavel Begunkov