Dave Martin <Dave.Martin@xxxxxxx> writes:
> On Thu, May 24, 2018 at 10:12:20AM +0200, Christoffer Dall wrote:
>> On Wed, May 23, 2018 at 08:35:13PM +0100, Alex Bennée wrote:
>> >
>> > Dave Martin <Dave.Martin@xxxxxxx> writes:
>> >
>> > > To make the lazy FPSIMD context switch trap code easier to hack on,
>> > > this patch converts it to C.
>> > >
>> > > This is not amazingly efficient, but the trap should typically only
>> > > be taken once per host context switch.
>> > >
>> > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
>> > > Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>> > > ---
>> > > arch/arm64/kvm/hyp/entry.S | 57 +++++++++++++++++----------------------------
>> > > arch/arm64/kvm/hyp/switch.c | 24 +++++++++++++++++++
>> > > 2 files changed, 46 insertions(+), 35 deletions(-)
>
> [...]
>
>> > > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>> > > index d964523..c0796c4 100644
>> > > --- a/arch/arm64/kvm/hyp/switch.c
>> > > +++ b/arch/arm64/kvm/hyp/switch.c
>> > > @@ -318,6 +318,30 @@ static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
>> > > }
>> > > }
>> > >
>> > > +void __hyp_text __hyp_switch_fpsimd(u64 esr __always_unused,
>> > > + struct kvm_vcpu *vcpu)
>> > > +{
>> > > + kvm_cpu_context_t *host_ctxt;
>> > > +
>> > > + if (has_vhe())
>> > > + write_sysreg(read_sysreg(cpacr_el1) | CPACR_EL1_FPEN,
>> > > + cpacr_el1);
>> > > + else
>> > > + write_sysreg(read_sysreg(cptr_el2) & ~(u64)CPTR_EL2_TFP,
>> > > + cptr_el2);
>> >
>> > Is there no way to do alternative() in C or does it always come down to
>> > different inline asms?
>> >
>>
>> has_vhe() should resolve to a static key, and I prefer this over the
>> previous alternative construct we had for selecting function calls in C,
>> as that resultet in having to follow too many levels of indirection.
>
> I'll defer to Christoffer on that -- I was just following precedent :)
>
> The if (has_vhe()) approach has the benefit of being much more
> readable, and the static branch predictor in many CPUs will succeed in
> folding a short-range unconditional branch out entirely. There will be
> a small increase in I-cache pressure due to the larger inline code
> size, but probably not much beyond that.
Fair enough - it was mostly a curiosity. It seems most of the use of
alternative() are mostly at the low level instruction level anyway.
--
Alex Bennée
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