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

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Message-ID: <087ca43a-49b7-40c9-915d-558075181fd1@gmail.com>
Date: Mon, 28 Jul 2025 19:58:13 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>, Byungchul Park <byungchul@...com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, horms@...nel.org, hawk@...nel.org, toke@...hat.com
Subject: Re: [RFC net-next] netmem: replace __netmem_clear_lsb() with
 netmem_to_nmdesc()

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@...com> 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