diff options
| author | Chang S. Bae <chang.seok.bae@intel.com> | 2023-01-20 16:18:57 -0800 |
|---|---|---|
| committer | Dave Hansen <dave.hansen@linux.intel.com> | 2023-03-22 13:02:18 -0700 |
| commit | ad9c29f3c29197aa25d26a5f258a98e4cb901996 (patch) | |
| tree | 8660b69558a7da77ce74cd4a25684e27884f8d0e | |
| parent | e8d018dd0257f744ca50a729e3d042cf2ec9da65 (diff) | |
Documentation/x86: Explain the purpose for dynamic features
This summary will help to guide the proper use of the enabling model.
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20230121001900.14900-2-chang.seok.bae%40intel.com
| -rw-r--r-- | Documentation/x86/xstate.rst | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst index 5cec7fb558d6..e954e79af4ce 100644 --- a/Documentation/x86/xstate.rst +++ b/Documentation/x86/xstate.rst @@ -11,6 +11,22 @@ are enabled by XCR0 as well, but the first use of related instruction is trapped by the kernel because by default the required large XSTATE buffers are not allocated automatically. +The purpose for dynamic features +-------------------------------- + +Legacy userspace libraries often have hard-coded, static sizes for +alternate signal stacks, often using MINSIGSTKSZ which is typically 2KB. +That stack must be able to store at *least* the signal frame that the +kernel sets up before jumping into the signal handler. That signal frame +must include an XSAVE buffer defined by the CPU. + +However, that means that the size of signal stacks is dynamic, not static, +because different CPUs have differently-sized XSAVE buffers. A compiled-in +size of 2KB with existing applications is too small for new CPU features +like AMX. Instead of universally requiring larger stack, with the dynamic +enabling, the kernel can enforce userspace applications to have +properly-sized altstacks. + Using dynamically enabled XSTATE features in user space applications -------------------------------------------------------------------- |