-----Original Message----- From: StephanT [mailto:stman937-linewbie@yahoo.com] Sent: Tuesday, October 18, 2011 6:01 PM To: Jeff Haran; kernelnewbies Subject: Re: How can I test if a physical address is already mapped or not.
Yes you are right when you are in user space. However in a kernel module this would be physical address.
Nope. Unless you are using some really strange processor that I am not familiar with, a memory read is a memory read. *pReg is going to read what's at virtual address 0xc0000000 regardless of whether you are executing in user or kernel context. I know it's going to work this way on a PC with an Intel x86 processor.
OK, but how do you explain I can read 0xC0000000 and I get kicked out if I try 0x00100000.
I can only assume that there is nothing mapped to virtual address 0x00100000. On a 32 bit PC, the bottom 3 GB of virtual memory is by default mapped to user space, so if anything you are accessing memory in the current process' virtual memory.
The 0xC0000000 falls under PCI address range. I guess on a PC Linux just doesn't map this address range or it maps it one2one (phys==virt) I can dereference this address and I get something plausible.
On most 32 bit Intel processors running "standard" Linux 0xC0000000 is the first address of kernel VM. This is a function of kernel configuration, but it's this way by default. It's still a virtual address. So yes, on your PC virtual address 0xC0000000 is mapped to physical RAM.
Would this mean by reading the 0xC0000000 I am reading the Linux code not PCI registers?
First part of the page table, I think. I don't really remember what the kernel maps down there but I don't think its code. Read the Gorman book. It's a lot to get through, but you'll likely learn a lot. There might even be something newer out there. Jeff Haran