linux-kernel - Re: [PATCH 14/26] drm/xe/eudebug: implement userptr

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Message-ID: <173444476233.58433.15197725556816943129@jlahtine-mobl.ger.corp.intel.com>
Date: Tue, 17 Dec 2024 16:12:42 +0200
From: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
To: Andrzej Hajda <andrzej.hajda@...el.com>, Christian König <christian.koenig@....com>, Christoph Hellwig <hch@....de>, Jonathan Cavitt <jonathan.cavitt@...el.com>, Linux MM <linux-mm@...ck.org>, Maciej Patelczyk <maciej.patelczyk@...el.com>, Mika Kuoppala <mika.kuoppala@...ux.intel.com>, dri-devel@...ts.freedesktop.org, intel-xe@...ts.freedesktop.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/26] drm/xe/eudebug: implement userptr_vma access

Quoting Joonas Lahtinen (2024-12-11 14:59:33)
> Quoting Christian König (2024-12-10 16:03:14)

<SNIP>

> > If you really want to expose an interface to userspace which walks the process
> > page table, installs an MMU notifier, kmaps the resulting page and then memcpy
> > to/from it then you absolutely *must* run that by guys like Christoph Hellwig,
> > Andrew and even Linus.

> > I'm pretty sure that those guys will note that a device driver should
> > absolutely not mess with such stuff.

<SNIP>

> >     But that seems like a high-overhead thing to do due to the overhead of
> >     setting up a transfer per data word and going over the PCI bus twice
> >     compared to accessing the memory directly by CPU when it trivially can.
> > 
> > 
> > Understandable, but that will create another way of accessing process memory.

Based on this feedback and some further discussion, we now have an alternative
implementation for this interface via access_process_vm function posted by Mika:

https://lore.kernel.org/dri-devel/20241216141721.2051279-1-mika.kuoppala@linux.intel.com/

It's a couple of dozen lines don't need to do any open-coded kmapping, only utilizing
the pre-existing memory access functions.

Hopefully that would address the above concerns?

Regards, Joonas

PS. It could still be optimized further to directly use the struct mm
from within the mm notifier, and go with access_remote_vm using that,
but would require new symbol export.

For demonstration it is implemented by grabbing the task_struct and using
the already exported access_process_vm function.