Re: kernel page table mapping for >1GB <3 GB for x86 arch without PAE

From: joe Shmoe
Date: Fri Aug 28 2009 - 17:07:11 EST

doesn't CR3 gets reloaded for process switch? If so , how does it matter if you setup page table mapping for all the available RAM instead of limiting to just 1GB

--- On Fri, 8/28/09, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:

> From: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
> Subject: Re: kernel page table mapping for >1GB <3 GB for x86 arch without PAE
> To: "joe Shmoe" <jsmoe3@xxxxxxxxx>
> Cc: Valdis.Kletnieks@xxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Date: Friday, August 28, 2009, 3:53 PM
> On Fri, 28 Aug 2009 12:44:25 -0700
> (PDT)
> joe Shmoe <jsmoe3@xxxxxxxxx>
> wrote:
> > No I am not talking about embedded systems. It is just
> an example.
> > My question is what is preventing the kernel from
> setting up the page tables so that entire RAM can be mapped
> upto 4 GB.
> The design of the x86 processor
> You only have 4GB of virtual address space at any one time
> so you have to
> fit the following within your 4GB virtual space
> - Virtual addresses of the user space
> - Mappings for physical memory
> - Kernel virtual spaces (vmalloc etc)
> - Various miscellaneous kernel mappings
> - I/O MMIO space
> Now you could in theory switch the page tables on kernel
> entry to get a
> very big user space mapping and then instead of relying on
> the user space
> being mapped you could map specific user pages or access
> them via their
> physical address lookup.
> That however means reloading %cr3 (page table base) which
> is very
> expensive, and some very hard to implement locking on user
> space access.
> Some other processor families have the notion of "spaces"
> or MMU
> bypassing. The Sparc64 for example can access MMIO space
> without an MMU
> mapping or virtual address being needed for it. x86 does
> not - it has
> segments but the segments map into the 4GB space so they
> don't help here
> hence the current implementation and 3G/1G split.

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