Shared VM address range across processes

John Giacomoni john.giacomoni at colorado.edu
Thu Mar 13 21:49:48 UTC 2008


Nikhil,

I am using kernel addresses to ensure that once the memory region is  
allocated
it is available to everyone, even for late joiners who might have an  
address
range conflict.

If you can predetermine or or otherwise simultaneously ensure the  
address range
is available for everyone life maybe easier :)  I haven't looked into  
it but
mapping userspace pages should be easier.

John G

On Mar 13, 2008, at 3:27 PM, Rao, Nikhil wrote:

>
> Hi John,
>
> Is the approach that you are working on based on necessarily using the
> kernel address space, so is this approach not feasible with user space
> virtual addresses ?
>
> Nikhil
>
> -----Original Message-----
> From: John Giacomoni [mailto:john.giacomoni at colorado.edu]
> Sent: Thursday, March 13, 2008 9:13 AM
> To: freebsd-hackers at freebsd.org
> Cc: Rao, Nikhil
> Subject: Re: Shared VM address range across processes
>
> Nihkil,
>
> I'm working on something similar for a research project and the answer
> is that it is possible but ugly.
>
> First, are you sure you need to do this?  Ensuring safety by checking
> pointers before dereferencing can be painful :)
>
> FreeBSD seems to have checks scattered throughout the kernel trying to
> ensure that the kernel address range remains unavailable to the
> userspace
> address range.  These checks can obviously be bypassed but they are
> fairly
> invasive.  Once all those checks are bipassed, you need to ensure that
> the
> PTEs and PDEs are have the userspace bit set for the appropriate page
> ranges which then requires flushing the specific pages out of the TLB
> using the invlpg function, note that flushing the TLB is  
> insufficient as
> kernel pages are marked global and thus won't flush with any other
> method.
>
> files that I touched
>
> /usr/src/sys/amd64/amd64/pmap.c  - pmap_enter
>
> /usr/src/sys/amd64/amd64/trap.c - trap_pfault
>
> and the allocation site needs to ensure that the user-mode bit is  
> set on
> the correct PTEs and PDEs.
>
> I directly allocate memory using vm objects to help me bypass the
> various
> address range checks that can be found in the higher levels of the
> kernel.
>
> I'm planning on generalizing and cleaning my approach up in the next  
> few
> months but I'll be glad to answer any specific questions you might  
> have.
>
>
> For the FreeBSD kernel developers,
>
> Is there a reason to enforce the high/low mem address range as  
> strongly
> as is done in FreeBSD?  It seems that if the higher-levels of the  
> kernel
> allow a mapping, the lower-levels should respect that.
>
> John G
>
>
>
> On Mar 12, 2008, at 12:46 PM, Rao, Nikhil wrote:
>
>> Hi,
>>
>>
>>
>> I want to map device memory into the same virtual address range in
>> multiple processes, this means I would have to add a vm_map_entry per
>> address range in every process, since the list of processes can be
>> potentially huge .. Is it allowed to point to the same list of
>> vm_map_entrys from multiple vm_spaces ? BSD3 had a field in the
>> vm_map_entry that could be a share map - would it be an idea that I
>> could reuse ?
>>
>>
>>
>> Nikhil
>>
>> _______________________________________________
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>
>
> --
>
> John.Giacomoni at colorado.edu
> University of Colorado at Boulder
> Department of Computer Science
> Engineering Center, ECCR 1B50
> 430 UCB
> Boulder, CO 80303-0430
> USA
>
> _______________________________________________
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> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
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--

John.Giacomoni at colorado.edu
University of Colorado at Boulder
Department of Computer Science
Engineering Center, ECCR 1B50
430 UCB
Boulder, CO 80303-0430
USA

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