On Thu, 2023-12-21 at 09:02 -0500, Yang Weijiang wrote: > Define a new XFEATURE_MASK_KERNEL_DYNAMIC mask to specify the features > that can be optionally enabled by kernel components. This is similar to > XFEATURE_MASK_USER_DYNAMIC in that it contains optional xfeatures that > can allows the FPU buffer to be dynamically sized. The difference is that > the KERNEL variant contains supervisor features and will be enabled by > kernel components that need them, and not directly by the user. Currently > it's used by KVM to configure guest dedicated fpstate for calculating > the xfeature and fpstate storage size etc. > > The kernel dynamic xfeatures now only contain XFEATURE_CET_KERNEL, which > is supported by host as they're enabled in kernel XSS MSR setting but > relevant CPU feature, i.e., supervisor shadow stack, is not enabled in > host kernel therefore it can be omitted for normal fpstate by default. > > 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. > > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx> > --- > arch/x86/include/asm/fpu/xstate.h | 5 ++++- > arch/x86/kernel/fpu/xstate.c | 1 + > 2 files changed, 5 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h > index 3b4a038d3c57..a212d3851429 100644 > --- a/arch/x86/include/asm/fpu/xstate.h > +++ b/arch/x86/include/asm/fpu/xstate.h > @@ -46,9 +46,12 @@ > #define XFEATURE_MASK_USER_RESTORE \ > (XFEATURE_MASK_USER_SUPPORTED & ~XFEATURE_MASK_PKRU) > > -/* Features which are dynamically enabled for a process on request */ > +/* Features which are dynamically enabled per userspace request */ > #define XFEATURE_MASK_USER_DYNAMIC XFEATURE_MASK_XTILE_DATA > > +/* Features which are dynamically enabled per kernel side request */ > +#define XFEATURE_MASK_KERNEL_DYNAMIC XFEATURE_MASK_CET_KERNEL > + > /* All currently supported supervisor features */ > #define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \ > XFEATURE_MASK_CET_USER | \ > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c > index 03e166a87d61..ca4b83c142eb 100644 > --- a/arch/x86/kernel/fpu/xstate.c > +++ b/arch/x86/kernel/fpu/xstate.c > @@ -824,6 +824,7 @@ void __init fpu__init_system_xstate(unsigned int legacy_size) > /* Clean out dynamic features from default */ > fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features; > fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC; > + fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_KERNEL_DYNAMIC; > > fpu_user_cfg.default_features = fpu_user_cfg.max_features; > fpu_user_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC; I still think that we should consider adding XFEATURE_MASK_CET_KERNEL to XFEATURE_MASK_INDEPENDENT or at least have a good conversation on why this doesn't make sense, but I also don't intend to fight over this, as long as the code works. Best regards, Maxim Levitsky
