On Wed, Oct 19, 2011 at 6:46 AM, Jeff Haran <jharan@xxxxxxxxxxxxxx> wrote: >> -----Original Message----- >> From: StephanT [mailto:stman937-linewbie@xxxxxxxxx] >> 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 > > > > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > Stephan, >From your post it looks like you are trying to simulate environment on Embedded Linux board to your PC? Did you check if embedded Linux has CONFIG_MMU kernel config option set? If that is the case then you might be bypassing page-table translation on your board and CPU assumes virtual address = physical address in that case. However same may not be true on PC which uses page-tables. "Understanding Linux Kernel" book is also a good reference for understanding memory management in Linux. One Advice: when shifting from one hardware to another, be ready for surprises and open to unlearn things :) Thanks, Rajat _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
