netdev - Re: af_packet.c flush_dcache

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Date:	Thu, 1 Nov 2007 19:27:57 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: af_packet.c flush_dcache_page

On Thu, Nov 01, 2007 at 05:10:32PM +0100, Patrick McHardy (kaber@...sh.net) wrote:
> David Miller wrote:
> >Instead of answering your questions, I'm going to show you
> >how to avoid having to do any of this cache flushing crap :-)
> >
> >You can avoid having to flush anything as long as the virtual
> >addresses on the kernel side are modulo SHMLBA the virtual addresses
> >on the userland side.
> >
> >We have some (decidedly awkward) mechanisms to try and achieve
> >this in the kernel, but they are cumbersome and not air tight.
> >
> >Instead, I would recommend simply that you access the ring
> >buffer directly in userspace.  This avoids all of the cache
> >aliasing issues.
> >
> >Yes, this means you have to do the ring buffer accesses in
> >the context of the user, but it simplifies so much that I think
> >it'd be worth it.
> 
> 
> I'm probably misunderstanding your suggestion because of my
> limited mm knowledge, are you suggesting to do something like
> this:
> 
> setsockopt(RX_RING, ...):
> 
> Allocate ring using get_user_pages, return address to user
> 
> tpacket_rcv/netlink_unicast/netlink_broadcast:
> 
> for each receiver:
> 	switch_mm(...)
> 	copy data to ring
> 
> switch_mm(original mm)
> 
> Would this work in softirq context?

IIRC it requires disabled interrupts.

Probably David suggests to provide a pointer to allocated
in userspace buffer and use copy_to_user() and friends.

> >Another option is to use the "copy_to_user_page()" and
> >"copy_from_user_page()" interfaces which will do all of
> >the necessary cache flushing for you.
> >
> >Actually it might be nice to convert AF_PACKET's mmap() code
> >over to using those things.
> 
> 
> That would also require to do the copy in the context of
> the user, right?

Most of the time it is possible to call copy_to_user() in atomic
context, but it can fail, in which case some additional mechanism to
make a copy should be invented (workqueue, kthread, whatever you like :)

-- 
	Evgeniy Polyakov
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