Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory

[John Hubbard]

On 10/26/20 6:28 AM, Matthew Wilcox wrote:
> On Sun, Oct 25, 2020 at 09:44:07PM -0700, John Hubbard wrote:
> > On 10/25/20 9:21 PM, Matthew Wilcox wrote:
> > > I don't think the page pinning approach is ever valid.  For file
> >
> > Could you qualify that? Surely you don't mean that the entire pin_user_pages
> > story is a waste of time--I would have expected you to make more noise
> > earlier if you thought that, yes?
>
> I do think page pinning is the wrong approach for everything.  I did say


Not *everything*, just "pinning for DMA", right? Because I don't recall
any viable solutions for Direct IO that avoided gup/pup!

Also, back to Case 5: I *could* create a small patchset to change over
the very few Case 5 call sites to use "gup, lock_page(), write to
page...etc", instead of pup. And also, update pin_user_pages.rst to
recommend that approach in similar situations. After all, it's not
really a long-term DMA pin, which is really what pin_user_pages*() is
intended for.

Would that be something you'd like to see happen? It's certainly easy
enough to fix that up. And your retroactive NAK is sufficient motivation
to do so.


> so at the time, and I continue to say so when the opportunity presents
> itself.  But shouting about it constantly only annoys people, so I don't
> generally bother.  I have other things to work on, and they're productive,
> so I don't need to spend my time arguing.


Sure. As a practical matter, I've assumed that page pinning is not going
to go away any time soon, so I want it to work properly while it's here.
But if there is a viable way to eventually replace dma-pinning with
something better, then let's keep thinking about it. I'm glad to help in
that area.


thanks,
--
John Hubbard
NVIDIA