Re: [Xen-devel] pagetable pinning question

[Bin Ren <br260@xxxxxxxxx>]

Hi, Zachary:

I can't describe how grateful I'm for your detailed, clear and 
professional answer!
Definitely a high-ranking in FAQ!!

Thanks so much!
-- Bin

On 6 Feb 2004, at 01:28, Zachary Amsden wrote:

> This is a good question, as all this terminology suffers from 
> potential overloading of the word "linear".  The linear recursive map 
> works as follows:
> Register %cr3 contains a physical page frame which points to the 
> current PDP.  This physical page contains the physical frame addresses 
> of all the second level page tables.  One of these entries is 
> special... it contains the physical frame address of the PDP itself.  
> The index of this entry now corresponds to the high 10 bits of a 
> virtual memory address.  Accesses to this 4Mb region of virtual memory 
> are translated through the PDP, which points to the actual (likely 
> non-contiguous) physical page tables.  So you have effectively mapped 
> the non-contiguous PTEs into the 4Mb contiguous (i.e. linear) virtual 
> memory region corresponding to the PDP index.  This greatly simplifies 
> some operations, since you can use the hardware to map the page tables 
> and access them as a giant array.  This is the linear map.
> The map is actually recursive, since the PDP itself will be mapped 
> into the same VM region.  So if your special index was say 767 (as in 
> NetBSD), then the 4Mb VM region at 0xbfc00000 would contain a linear 
> mapping of your page tables.  The page at 0xbfeff000 would contain the 
> PDP itself, and the virtual address 0xbfeffbfc would contain the 
> physical address of the PDP.
> You can also use other slots in the PDP to map other processes page 
> tables, which might be useful for shared memory access or implementing 
> fork().  This is what causes the problems with reference accounting, 
> since you have the same page being used both as a PDP and a PTP, with 
> possible circular references.
> Zachary Amsden
> zamsden@xxxxxxxxx
>
> Bin Ren wrote:
>
> > Hi,
> >
> > I'm a bit confused by parts of the discussion going here. Can anyone
> > explain the difference between linear page table and normal page 
> > table?
> > Linux page tables are all linear, i.e. contiguous page table entries
> > correspond to contiguous virtual pages. Is this right? How about
> > NetBSD?
> >
> > How can one page directory be mapped from another page directory?
> >
> > My understanding is: (maybe wrong) each process has only one
> > page directory and 1024 page tables. Each page directory and page
> > table fits into one page (on 32-bit computers with 4k page size).
> > Not all page tables are allocated physical page frames initially. 
> > During
> > a context switch from process A to process B, A's PD and PTs can be
> > swapped out (in face of memory shortage), releasing physical page
> > frames for use by B. So, how can one PD be mapped by another PD?
> >
> > What am I missing here?
> >
> > Thanks,
> > Bin
> >
> >
> >
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