From mboxrd@z Thu Jan  1 00:00:00 1970
References: <20220523140444.2632658-1-guntgrau@bbl.ma.philips.com>
 <59e12ca2-6c6f-09ff-5365-985586dc4e4e@siemens.com>
From: Philippe Gerum <rpm@xenomai.org>
Subject: Re: [PATCH] rtdm/drvlib: Prevent pagefaults on arm on io mapping
Date: Wed, 15 Jun 2022 09:54:23 +0200
In-reply-to: <59e12ca2-6c6f-09ff-5365-985586dc4e4e@siemens.com>
Message-ID: <87fsk6pddt.fsf@xenomai.org>
MIME-Version: 1.0
Content-Type: text/plain
List-Id: Discussions about the Xenomai project <xenomai.xenomai.org>
List-Unsubscribe: <https://xenomai.org/mailman/options/xenomai>,
 <mailto:xenomai-request@xenomai.org?subject=unsubscribe>
List-Archive: <http://xenomai.org/pipermail/xenomai/>
List-Post: <mailto:xenomai@xenomai.org>
List-Help: <mailto:xenomai-request@xenomai.org?subject=help>
List-Subscribe: <https://xenomai.org/mailman/listinfo/xenomai>,
 <mailto:xenomai-request@xenomai.org?subject=subscribe>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Gunter Grau <gunter.grau@philips.com>, Gunter Grau <guntgrau@bbl.ma.philips.com>, xenomai@xenomai.org


Jan Kiszka via Xenomai <xenomai@xenomai.org> writes:

> On 23.05.22 16:04, Gunter Grau via Xenomai wrote:
>> From: Gunter Grau <gunter.grau@philips.com>
>> 
>> When mapping io memory into userspace an extra simulated pagefault for all
>> pages is added to prevent later pagefaults because of copy on write
>> mechanisms. This happens only on architectures that have defined the
>> needed cobalt_machine.prefault function.
>> 
>> Signed-off-by: Gunter Grau <gunter.grau@philips.com>
>> ---
>>  kernel/cobalt/rtdm/drvlib.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/cobalt/rtdm/drvlib.c b/kernel/cobalt/rtdm/drvlib.c
>> index 4eaf3a57c..db8431ee1 100644
>> --- a/kernel/cobalt/rtdm/drvlib.c
>> +++ b/kernel/cobalt/rtdm/drvlib.c
>> @@ -1761,6 +1761,7 @@ static int mmap_iomem_helper(struct vm_area_struct *vma, phys_addr_t pa)
>>  {
>>  	pgprot_t prot = PAGE_SHARED;
>>  	unsigned long len;
>> +	int ret;
>>  
>>  	len = vma->vm_end - vma->vm_start;
>>  #ifndef CONFIG_MMU
>> @@ -1774,8 +1775,15 @@ static int mmap_iomem_helper(struct vm_area_struct *vma, phys_addr_t pa)
>>  #endif
>>  	vma->vm_page_prot = pgprot_noncached(prot);
>>  
>> -	return remap_pfn_range(vma, vma->vm_start, pa >> PAGE_SHIFT,
>> +	ret = remap_pfn_range(vma, vma->vm_start, pa >> PAGE_SHIFT,
>>  			       len, vma->vm_page_prot);
>> +	if (ret)
>> +		return ret;
>> +
>> +	if (cobalt_machine.prefault)
>> +		cobalt_machine.prefault(vma);
>> +
>> +	return ret;
>>  }
>>  
>>  static int mmap_buffer_helper(struct rtdm_fd *fd, struct vm_area_struct *vma)
>
> Wow, that was likely broken by the refactoring in c8e9e166, long ago.
>
> Applied to next
>

The prefault hook has always been specifically about COW-breaking, I/O
memory has no business with this, so there is no point in having the
iomem helper calling the prefaulting hook.

I suspect that rtdm_mmap_to_user() should be called instead of
rtdm_iomap_to_user() in the case at hand.

-- 
Philippe.