Re: [PATCH v10 02/30] arm64/fpsimd: Update FA64 and ZT0 enables when loading SME state

[Catalin Marinas]

On Fri, Mar 06, 2026 at 05:00:54PM +0000, Mark Brown wrote:
> Currently we enable EL0 and EL1 access to FA64 and ZT0 at boot and leave
> them enabled throughout the runtime of the system. When we add KVM support
> we will need to make this configuration dynamic, these features may be
> disabled for some KVM guests. Since the host kernel saves the floating
> point state for non-protected guests and we wish to avoid KVM having to
> reload the floating point state needlessly on guest reentry let's move the
> configuration of these enables to the floating point state reload.
> 
> We provide a helper which does the configuration as part of a
> read/modify/write operation along with the configuration of the task VL,
> then update the floating point state load and SME access trap to use it.
> We also remove the setting of the enable bits from the CPU feature
> identification and resume paths.  There will be a small overhead from
> setting the enables one at a time but this should be negligible in the
> context of the state load or access trap.  In order to avoid compiler
> warnings due to unused variables in !CONFIG_ARM64_SME cases we avoid
> storing the vector length in temporary variables.
> 
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>