Add documentation updates that capture PowerPC specific changes. Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx> --- Documentation/vm/protection-keys.txt | 65 +++++++++++++++++++++++++----------- 1 file changed, 45 insertions(+), 20 deletions(-) diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt index b643045..965ad75 100644 --- a/Documentation/vm/protection-keys.txt +++ b/Documentation/vm/protection-keys.txt @@ -1,21 +1,46 @@ -Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature -which will be found on future Intel CPUs. +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found in +new generation of intel CPUs and on PowerPC 7 and higher CPUs. Memory Protection Keys provides a mechanism for enforcing page-based -protections, but without requiring modification of the page tables -when an application changes protection domains. It works by -dedicating 4 previously ignored bits in each page table entry to a -"protection key", giving 16 possible keys. - -There is also a new user-accessible register (PKRU) with two separate -bits (Access Disable and Write Disable) for each key. Being a CPU -register, PKRU is inherently thread-local, potentially giving each -thread a different set of protections from every other thread. - -There are two new instructions (RDPKRU/WRPKRU) for reading and writing -to the new register. The feature is only available in 64-bit mode, -even though there is theoretically space in the PAE PTEs. These -permissions are enforced on data access only and have no effect on +protections, but without requiring modification of the page tables when an +application changes protection domains. + + +On Intel: + + It works by dedicating 4 previously ignored bits in each page table + entry to a "protection key", giving 16 possible keys. + + There is also a new user-accessible register (PKRU) with two separate + bits (Access Disable and Write Disable) for each key. Being a CPU + register, PKRU is inherently thread-local, potentially giving each + thread a different set of protections from every other thread. + + There are two new instructions (RDPKRU/WRPKRU) for reading and writing + to the new register. The feature is only available in 64-bit mode, + even though there is theoretically space in the PAE PTEs. These + permissions are enforced on data access only and have no effect on + instruction fetches. + + +On PowerPC: + + It works by dedicating 5 page table entry bits to a "protection key", + giving 32 possible keys. + + There is a user-accessible register (AMR) with two separate bits; + Access Disable and Write Disable, for each key. Being a CPU + register, AMR is inherently thread-local, potentially giving each + thread a different set of protections from every other thread. NOTE: + Disabling read permission does not disable write and vice-versa. + + The feature is available on 64-bit HPTE mode only. + 'mtspr 0xd, mem' reads the AMR register + 'mfspr mem, 0xd' writes into the AMR register. + + + +Permissions are enforced on data access only and have no effect on instruction fetches. =========================== Syscalls =========================== @@ -28,9 +53,9 @@ There are 3 system calls which directly interact with pkeys: unsigned long prot, int pkey); Before a pkey can be used, it must first be allocated with -pkey_alloc(). An application calls the WRPKRU instruction +pkey_alloc(). An application calls the WRPKRU/AMR instruction directly in order to change access permissions to memory covered -with a key. In this example WRPKRU is wrapped by a C function +with a key. In this example WRPKRU/AMR is wrapped by a C function called pkey_set(). int real_prot = PROT_READ|PROT_WRITE; @@ -52,11 +77,11 @@ is no longer in use: munmap(ptr, PAGE_SIZE); pkey_free(pkey); -(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions. +(Note: pkey_set() is a wrapper for the RDPKRU,WRPKRU or AMR instructions. An example implementation can be found in tools/testing/selftests/x86/protection_keys.c) -=========================== Behavior =========================== +=========================== Behavior ================================= The kernel attempts to make protection keys consistent with the behavior of a plain mprotect(). For instance if you do this: -- 1.8.3.1
