linux-kernel - Re: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers

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Message-ID: <c0b91a50-d3e7-44f9-b9c5-9c3b29639428@suse.de>
Date: Tue, 3 Jun 2025 08:25:24 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Michael Kelley <mhklinux@...look.com>,
 David Hildenbrand <david@...hat.com>, "simona@...ll.ch" <simona@...ll.ch>,
 "deller@....de" <deller@....de>,
 "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
 "kys@...rosoft.com" <kys@...rosoft.com>,
 "wei.liu@...nel.org" <wei.liu@...nel.org>,
 "decui@...rosoft.com" <decui@...rosoft.com>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc: "weh@...rosoft.com" <weh@...rosoft.com>, "hch@....de" <hch@....de>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel
 memory framebuffers

Hi

Am 03.06.25 um 03:49 schrieb Michael Kelley:
[...]
>> Will the VMA have VM_PFNMAP or VM_MIXEDMAP set? PFN_SPECIAL is a
>> horrible hack.
>>
>> In another thread, you mention that you use PFN_SPECIAL to bypass the
>> check in vm_mixed_ok(), so VM_MIXEDMAP is likely not set?
> The VMA has VM_PFNMAP set, not VM_MIXEDMAP.  It seemed like
> VM_MIXEDMAP is somewhat of a superset of VM_PFNMAP, but maybe that's
> a wrong impression. vm_mixed_ok() does a thorough job of validating the
> use of __vm_insert_mixed(), and since what I did was allowed, I thought
> perhaps it was OK. Your feedback has set me straight, and that's what I
> needed. :-)
>
> But the whole approach is moot with Alistair Popple's patch set that
> eliminates pfn_t. Is there an existing mm API that will do mkwrite on a
> special PTE in a VM_PFNMAP VMA? I didn't see one, but maybe I missed
> it. If there's not one, I'll take a crack at adding it in the next version of my
> patch set.

What is the motivation behind this work? The driver or fbdev as a whole 
does not have much of a future anyway.

I'd like to suggest removing hyperv_fb entirely in favor of hypervdrm?

Best regards
Thomas

>
> Michael

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)