The following commit has been merged into the x86/fpu branch of tip: Commit-ID: ad9c29f3c29197aa25d26a5f258a98e4cb901996 Gitweb: https://git.kernel.org/tip/ad9c29f3c29197aa25d26a5f258a98e4cb901996 Author: Chang S. Bae <chang.seok.bae@xxxxxxxxx> AuthorDate: Fri, 20 Jan 2023 16:18:57 -08:00 Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> CommitterDate: Wed, 22 Mar 2023 13:02:18 -07:00 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@xxxxxxxxx> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx> Link: https://lore.kernel.org/all/20230121001900.14900-2-chang.seok.bae%40intel.com --- Documentation/x86/xstate.rst | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst index 5cec7fb..e954e79 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 --------------------------------------------------------------------
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