On Thu, May 22, 2025 at 08:10:04AM -0700, Chao Gao wrote: > Currently, guest and host FPUs share the same default features. However, > the CET supervisor xstate is the first feature that needs to be enabled > exclusively for guest FPUs. Enabling it for host FPUs leads to a waste of > 24 bytes in the XSAVE buffer. > > To support "guest-only" features, add a new structure to hold the > default features and sizes for guest FPUs to clearly differentiate them > from those for host FPUs. > > Add two helpers to provide the default feature masks for guest and host > FPUs. Default features are derived by applying the masks to the maximum > supported features. > > Note that, > 1) for now, guest_default_mask() and host_default_mask() are identical. > This will change in a follow-up patch once guest permissions, default > xfeatures, and fpstate size are all converted to use the guest defaults. > > 2) only supervisor features will diverge between guest FPUs and host > FPUs, while user features will remain the same [1][2]. So, the new > vcpu_fpu_config struct does not include default user features and size > for the UABI buffer. > > An alternative approach is adding a guest_only_xfeatures member to > fpu_kernel_cfg and adding two helper functions to calculate the guest > default xfeatures and size. However, calculating these defaults at runtime > would introduce unnecessary overhead. > > Suggested-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx> Reviewed-by: John Allen <john.allen@xxxxxxx>
