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:[EMAIL PROTECTED]
Sent: Thursday, March 13, 2008 9:13 AM
To: [email protected]
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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Department of Computer Science
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USA
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