Re: [PATCH v13 02/32] arm64/fpsimd: Ensure all of ZCR_EL1 is initialised from idle

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jul 23, 2026 at 01:58:11PM +0100, Mark Rutland wrote:
> On Mon, Jul 20, 2026 at 12:07:29AM +0100, Mark Brown wrote:
> > At present when exiting from idle we do not fully reinitialise ZCR_EL1,
> > we update ZCR_EL1.LEN with a read/modify/write cycle when loading task
> > state but never set any of the other bits to an explicit value.  Since
> > currently they are all architecturally RES0 or RAZ/WI this is not a
> > practical issue but it may become one if further fields are defined in
> > the register so we should explicitly configure the whole register.
> >
> > Rename the existing sme_suspend_exit() (which handles this for SME) to
> > fpsimd_suspend_exit() and add set ZCR_EL1 to 0 there, if needed LEN will
> > be updated when loading task state.
> >
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> 
> While this happens to work today, this is a more general architecture
> problem, and I think we should cc stable such that kernels will work
> reliably on future hardware.
> 
> All stable kernels support SVE, so this needs to go as far back as
> v5.10.y.
> 
> One minor comment below, but with that fixed up (and a CC stable):
> 
> Acked-by: Mark Rutland <mark.rutland@arm.com>

Sorry, having looked at some of the later patches I think this isn't
quite right even with those changes, as we wouldn't have necessary
context synchronization in all cases.

I'll need to dig a bit more to come up with a more concrete solution.

Mark.

> > +void fpsimd_suspend_exit(void)
> >  {
> >  	u64 smcr = 0;
> >  
> > -	if (!system_supports_sme())
> > -		return;
> > +	if (system_supports_sve())
> > +		write_sysreg_s(0, SYS_ZCR_EL1);
> >  
> > -	if (system_supports_fa64())
> > -		smcr |= SMCR_ELx_FA64;
> > -	if (system_supports_sme2())
> > -		smcr |= SMCR_ELx_EZT0;
> > +	if (system_supports_sme()) {
> 
> We should move the 'smcr' variable into this block. That way it's scoped
> to where it matters.
> 
> > +		if (system_supports_fa64())
> > +			smcr |= SMCR_ELx_FA64;
> > +		if (system_supports_sme2())
> > +			smcr |= SMCR_ELx_EZT0;
> >  
> > -	write_sysreg_s(smcr, SYS_SMCR_EL1);
> > -	write_sysreg_s(0, SYS_SMPRI_EL1);
> > +		write_sysreg_s(smcr, SYS_SMCR_EL1);
> > +		write_sysreg_s(0, SYS_SMPRI_EL1);
> > +	}
> >  }
> 
> Mark.
>