Re: [PATCH v8 13/38] KVM: arm64: Manage GCS registers for guests

[Mark Brown <broonie@kernel.org>]

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On Mon, Feb 05, 2024 at 09:46:16AM +0000, Marc Zyngier wrote:
> On Sat, 03 Feb 2024 12:25:39 +0000,
> Mark Brown <broonie@kernel.org> wrote:

> > +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> > @@ -25,6 +25,8 @@ static inline void __sysreg_save_user_state(struct kvm_cpu_context *ctxt)
> >  {
> >  	ctxt_sys_reg(ctxt, TPIDR_EL0)	= read_sysreg(tpidr_el0);
> >  	ctxt_sys_reg(ctxt, TPIDRRO_EL0)	= read_sysreg(tpidrro_el0);
> > +	if (has_gcs())
> > +		ctxt_sys_reg(ctxt, GCSPR_EL0) = read_sysreg_s(SYS_GCSPR_EL0);

> We have had this discussion in the past. This must be based on the
> VM's configuration. Guarding the check with the host capability is a
> valuable optimisation, but that's nowhere near enough. See the series
> that I have posted on this very subject (you're on Cc), but you are
> welcome to invent your own mechanism in the meantime.

Right, which postdates the version you're replying to and isn't merged
yet - the current code was what you were asking for at the time.  I'm
expecting to update all these feature series to work with that once it
gets finalised and merged but it's not there yet, I do see I forgot to
put a note in v9 about that like I did for dpISA - sorry about that, I
was too focused on the clone3() rework when rebasing onto the new
kernel.

This particular series isn't going to get merged for a while yet anyway
due to the time it'll take for userspace testing, I'm expecting your
series to be in by the time it becomes an issue.

> > +	if (has_gcs()) {
> > +		write_sysreg_el1(ctxt_sys_reg(ctxt, GCSPR_EL1),	SYS_GCSPR);
> > +		write_sysreg_el1(ctxt_sys_reg(ctxt, GCSCR_EL1),	SYS_GCSCR);
> > +		write_sysreg_s(ctxt_sys_reg(ctxt, GCSCRE0_EL1),
> > +			       SYS_GCSCRE0_EL1);
> > +	}

> For the benefit of the unsuspecting reviewers, and in the absence of a
> public specification (which the XML drop isn't), it would be good to
> have the commit message explaining the rationale of what gets saved
> when.

What are you looking for in terms of rationale here?  The KVM house
style is often very reliant on reader context so it would be good to
know what considerations you'd like to see explicitly addressed.  These
registers shouldn't do anything when we aren't running the guest so
they're not terribly ordering sensitive, the EL2 ones will need a bit
more consideration in the face of nested virt.

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