March 2013 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

64bit MMIO access
by Jagath Weerasinghe March 11, 2013

March 11, 2013

Hi, What are the read/write kernel functions to access 64bit MMIO addresses? Are there similar functions like readl() and writel() for 64bit systems? Regards, Jagath

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Userspace interception of locally valid memory location
by harish badrinath March 10, 2013

March 10, 2013

Hello, Is it possible to intercept (both read and write) a locally valid address of a process and replace it with our own values (it is for a transparent distributed shared memory project). Regards, Harish Badrinath

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Disabling interrupts and masking interrupts
by Kevin Wilson March 9, 2013

March 9, 2013

Hello, what is the difference between disabling interrupts and masking interrupts ? Disabling interrupts is done, AFAIK, with irq_disable(). (see below) Can someone gives an example of how to mask interrupts with x86/x86_64 ? irq_disable() in x86 goes to native_irq_disable(), which eventually calls assembler "cli" command: CLI clears the IF bit in the flags. static inline void native_irq_disable(void) { asm volatile("cli": : :"memory"); } see arch/x86/include/asm/irqflags.h rgs, Kevin

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TLB shootdowns in /proc/interrupts (different results)
by Kevin Wilson March 9, 2013

March 9, 2013

Hi, I have two x86_64 machines: On the first there is Fedora 18 (x86_64); on the second there is Ubunutu 12.10 (server, 64 bit). I run cat /proc/interrupts on both machines; in both cases, I run this command after an hour when the machine is up. I get: On the fedora machine cat /proc/interrupts | grep TLB TLB: 0 0 TLB shootdowns On the Ubuntu machine, however, there are many TLB shootdowns, on both cores. Can anyone explains why is it so ? rgs, Kevin

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help with page_fault
by ishare March 9, 2013

March 9, 2013

In boot stage , my kernel get into loop restart due to page_fault. I have catched the source . here is the backtrace : #3 0xc01c686e in new_inode (sb=0xc8df5800) at fs/inode.c:551 #4 0xc01d076d in get_sb_pseudo (fs_type=0xc02e1d00, name=0xc02c3747 "futex", ops=0x0, magic=195894762) at fs/libfs.c:201 #5 0xc015cbbf in futexfs_get_sb (fs_type=0xc02e1d00, flags=0, dev_name=0xc02c374d "futexfs", data=0x0) at kernel/futex.c:679 #6 0xc01a62c5 in do_kern_mount (fstype=0xc02c374d "futexfs", flags=0, name=0xc02c374d "futexfs", data=0x0) at fs/super.c:718 code at the frame 3 is : struct inode *root = new_inode(s); after return from the new_inode , the root->i_dentry=(0x1,0x3). then calling the function d_instantiate(entry, root),here entry is of type struct entry*, this calls do list_add(&entry->d_alias, root->i_dentry), Obviously,this need to address the inode->i_dentry->next,which is ox3, but 0x3 is low address ,addressing to it causes a page_fault ,then make system restart . I doubt that the returned root inode need to be initialized using a applicable i_dentry . If so ,what value can be used ? If not , where am I wrong? thanks!

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Will kernel in-place decompression overwrite uncompressed part ?
by Jacky March 9, 2013

March 9, 2013

Dear All, Before, kernel first decompressed to other memory location and then merged and put at final destination. Now, kernel decompression are decompressed in-place, so-called in-place decompression. But the compressed part is encompassed by uncompressed part. So, after decompression, will the uncompressed part be overwritten by the decompressed kernel ? If so, some uncompressed part such as deal with kernel relocation will be overwritten by the decompressed kernel, which don't make sense, am I missed something ? Regards, Jacky

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Why FIB_TABLE_HASHSZ=2 when CONFIG_IP_ROUTE_MULTIPATH
by tingwei liu March 8, 2013

March 8, 2013

Hi, all who can tell me why? I think is should be 256 also. #ifdef CONFIG_IP_ROUTE_MULTIPATH #define FIB_RES_NH(res) ((res).fi->fib_nh[(res).nh_sel]) #define FIB_TABLE_HASHSZ 2 #else /* CONFIG_IP_ROUTE_MULTIPATH */ #define FIB_RES_NH(res) ((res).fi->fib_nh[0]) #define FIB_TABLE_HASHSZ 256 #endif /* CONFIG_IP_ROUTE_MULTIPATH */ Thanks

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What is the memory region ?
by Jacky March 8, 2013

March 8, 2013

Dear all, This is the Program Header for "cat" info: ================================ readelf -l /bin/cat ... Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align PHDR 0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4 INTERP 0x000154 0x08048154 0x08048154 0x00013 0x00013 R 0x1 [Requesting program interpreter: /lib/ld-linux.so.2] LOAD 0x000000 0x08048000 0x08048000 0x0a00c 0x0a00c R E 0x1000 LOAD 0x00af04 0x08053f04 0x08053f04 0x00258 0x00820 RW 0x1000 DYNAMIC 0x00af10 0x08053f10 0x08053f10 0x000e8 0x000e8 RW 0x4 NOTE 0x000168 0x08048168 0x08048168 0x00044 0x00044 R 0x4 GNU_EH_FRAME 0x009008 0x08051008 0x08051008 0x002d4 0x002d4 R 0x4 GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4 GNU_RELRO 0x00af04 0x08053f04 0x08053f04 0x000fc 0x000fc R 0x1 ============================== So there are just 2 PT_LOAD segments. But why kernel maps 3 memory regions ? The following is the maps output: ============================ cat /proc/self/maps 08048000-08053000 r-xp 00000000 08:01 261656 /bin/cat 08053000-08054000 r--p 0000a000 08:01 261656 /bin/cat 08054000-08055000 rw-p 0000b000 08:01 261656 /bin/cat 09b58000-09b79000 rw-p 00000000 00:00 0 [heap] b75bd000-b75be000 rw-p 00000000 00:00 0 b75be000-b7761000 r-xp 00000000 08:01 523958 /lib/i386-linux-gnu/libc-2.15.so ... ================== The above output, there are 3 memory regions for "/bin/cat", and what is the following segment: 08053000-08054000 r--p 0000a000 08:01 261656 /bin/cat According the 'cat' program header, there is no "r" segment. Regards, Jacky

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zap_low_mappings
by ishare March 7, 2013

March 7, 2013

kernel halts because the page mapping has been modified by zap_low_mappings , why we should do zap_low_mappings in init procedure ? this will disorder the page mapping. thanks!

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Several unrelated beginner questions.
by Konstantin Kowalski March 7, 2013

March 7, 2013

Hello everyone, I am interested in Linux kernel programming (and OS kernels and general), and I am currently reading several books about Linux kernel. I have a few questions about it: 1.) Currently, I am reading 2 books about Linux kernel: Linux Device Drivers (3rd edition) and Linux Kernel Development (3rd edition). I like both books and I am learning a lot from them. I heard that both of this books are outdated, but so far all the information in this books seems valid and applicable. Is there better books you would recommend? 2.) In Linux Device Drivers, it states that module_exit(function) is discarded if module is built directly into kernel or if kernel is compiled with option to disallow loadable modules. But what if the module still has to do something during shutdown? Releasing memory is unimportant since it does not persist over reboot, but what if the module has to write something to a disk file, or do some other action? 3.) What's the deal with different kernel versions? I heard back in the 2.x days, even kernels were stable and odd versions were experimental, but with 2.6 it changed. So with 3.x kernels, are all of them experimental in the beginning and stable in the end? Also, with 3.x new versions seem to be released more often than in 2.1-2.5 days. Did the release cycle get smaller or is it just my imagination? Also, what does rc number mean? 4.) Currently, I am running linux-next, and it works great. Am I correct to assume that linux-next is supposed to have newest, shiniest and most unstable features? `uname -a` says that I am still running 3.8-next, but there is already 3.9 out. So which version is more experimental and least stable? Which one is the newest? 5.) How exactly does make/.config work? When I run `make oldconfig`, does it use the everything from the previous .config and only ask how to configure new features? And when I run `make` does it re-use old object files if nothing was changed in the specific file, or does it re-compile everything from scratch? Thank you, Kostyantyn Kovalskyy (Konstantin Kowalski)

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