From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex =?utf-8?Q?Benn=C3=A9e?= Subject: Re: [PATCH v10 05/18] KVM: arm64: Convert lazy FPSIMD context switch trap to C Date: Thu, 24 May 2018 10:14:46 +0100 Message-ID: <87fu2htny1.fsf@linaro.org> References: <1527005119-6842-1-git-send-email-Dave.Martin@arm.com> <1527005119-6842-6-git-send-email-Dave.Martin@arm.com> <87k1rutbbi.fsf@linaro.org> <20180524081220.GJ55598@C02W217FHV2R.local> <20180524085445.GP13470@e103592.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: In-reply-to: <20180524085445.GP13470@e103592.cambridge.arm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Dave Martin Cc: Christoffer Dall , Ard Biesheuvel , Marc Zyngier , Catalin Marinas , Will Deacon , Christoffer Dall , kvmarm@lists.cs.columbia.edu, 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<87k1rutbbi.fsf@linaro.org> <20180524081220.GJ55598@C02W217FHV2R.local> <20180524085445.GP13470@e103592.cambridge.arm.com> Message-ID: <87fu2htny1.fsf@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Dave Martin 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 Benn?e wrote: >> > >> > Dave Martin 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 >> > > Reviewed-by: Marc Zyngier >> > > --- >> > > 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 Benn?e