So basically you use denied permissions to trap the access, and then emulate the access bits yourself?
On Thu, Aug 13, 2015 at 7:19 PM, Kevin Elphinstone < [email protected]> wrote: > The classic paper on dealing with paging without access to reference bits > is: > > > > “*Converting a swap-based system to do paging in an architecture lacking > page-referenced bits”* > > http://dl.acm.org/citation.cfm?id=806595 > > > > Note: Basically, hardware dirty/reference bits are really only a > performance optimisation. > > > > You still have to implement page-based virtual memory on top of seL4, i.e. > something that provides virtual memory and paging to external storage, for > whatever your applications are, etc…. > > > > Given an implementation of page-based virtual memory (and a replacement > algorithm such as “2nd chance clock”), the difference between having > access to hardware bits and simulating bits is pretty minor. > > > > We can look into supporting querying the hardware bits, but lack of access > to them is not preventing building a specific system due to ease of > emulation. > > > > - Kevin > > > > > > > > *From:* Devel [mailto:[email protected]] *On Behalf Of *Raymond > Jennings > *Sent:* Friday, 14 August 2015 7:21 AM > *To:* Adrian Danis <[email protected]> > *Cc:* [email protected] > *Subject:* Re: [seL4] Detecting dirty/accessed page tables > > > > So to be specific on the whole accessed/dirty bit topic. > > > > My first intention was to ask how to do it presently, if possible. > > > > When I found out it wasn't...I'd like to make it a suggestion for a future > version. > > > > I'm brand new to this list, so is this the proper way to make suggetions? > > > > On Thu, Aug 13, 2015 at 1:47 PM, Raymond Jennings <[email protected]> > wrote: > > On Thu, Aug 13, 2015 at 5:48 AM, Adrian Danis <[email protected]> > wrote: > > Currently there is no support in seL4 for taking advantage of hardware > support for detecting dirty/accessed pages. If you want this then you must > emulate this in software by changing the access writes on pages and > handling the resulting page faults. > > The reason for not supporting any underlying hardware features for this is > the original, and verified, platform that seL4 was written, an armv6, did > not have hardware support for tracking this. > > Since porting seL4 to x86 and newer ARM architectures that have support > for this there has not been any perceived need for this, so it has not been > done. There is certainly no reason requesting such information could not be > done within the capability model. > > > > This is why I was asking if having mapping rights on the page > descriptor/page table would be sufficient. If you can change which > physical page a virtual address refers to, you could probably also have > enough privilege to be trusted with dirty/accessed bit. > > > > Is this the sort of "within the capability model" you are referring to? > > > > > > ------------------------------ > > The information in this e-mail may be confidential and subject to legal > professional privilege and/or copyright. National ICT Australia Limited > accepts no liability for any damage caused by this email or its attachments. >
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