On Tue, Mar 04, 2025 at 02:37:00PM -0800, Dave Hansen wrote: >Subjects should ideally be written without large identifiers: > > Subject: x86/fpu/xstate: Introduce dynamic kernel features will do. > >On 11/26/24 02:17, Chao Gao wrote: >> Remove the kernel dynamic feature from fpu_kernel_cfg.default_features >> so that the bits in xstate_bv and xcomp_bv are cleared and xsaves/xrstors >> can be optimized by HW for normal fpstate. > >I'm really confused why this changelog hunk is here. > >Right now, all kernel XSAVE buffers have the same supervisor xfeatures. >This introduces the idea that they can have different xfeature sets. > >The _purpose_ of this is to save space when only some FPUs need a given >feature. This saves space in 'struct fpu'. It probably doesn't actually >make XSAVE/XRSTOR any faster because the init optimization is very >likely to be in place for these features. Thanks for the detailed explanation. You're absolutely right—this change aims to reduce the size of struct fpu. I see now that implying a perf improvement for XSAVE/XRSTOR was misleading, as the underlying optimization is likely handled by the init optimization. I'll revise the changelog to clarify this and incorporate your wording to avoid confusion. > >I'm not sure what point the changelog was trying to make about xstate_bv >and xcomp_bv. xstate_bv[12] would have been 0 if the feature was in its >init state.
