From: alex.bennee@linaro•org (Alex Bennée)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v10 05/18] KVM: arm64: Convert lazy FPSIMD context switch trap to C
Date: Thu, 24 May 2018 10:14:46 +0100 [thread overview]
Message-ID: <87fu2htny1.fsf@linaro.org> (raw)
In-Reply-To: <20180524085445.GP13470@e103592.cambridge.arm.com>
Dave Martin <Dave.Martin@arm•com> 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 <Dave.Martin@arm•com> 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@arm•com>
>> > > Reviewed-by: Marc Zyngier <marc.zyngier@arm•com>
>> > > ---
>> > > 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
next prev parent reply other threads:[~2018-05-24 9:14 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-22 16:05 [PATCH v10 00/18] KVM: arm64: Optimise FPSIMD context switching Dave Martin
2018-05-22 16:05 ` [PATCH v10 01/18] arm64: fpsimd: Fix TIF_FOREIGN_FPSTATE after invalidating cpu regs Dave Martin
2018-05-23 11:33 ` Christoffer Dall
2018-05-23 13:44 ` Alex Bennée
2018-05-23 13:46 ` Catalin Marinas
2018-05-22 16:05 ` [PATCH v10 02/18] thread_info: Add update_thread_flag() helpers Dave Martin
2018-05-23 13:46 ` Alex Bennée
2018-05-23 13:57 ` Dave Martin
2018-05-23 14:35 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 03/18] arm64: Use update{,_tsk}_thread_flag() Dave Martin
2018-05-23 13:48 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 04/18] KVM: arm/arm64: Introduce kvm_arch_vcpu_run_pid_change Dave Martin
2018-05-23 14:34 ` Alex Bennée
2018-05-23 14:40 ` Dave Martin
2018-05-24 8:11 ` Christoffer Dall
2018-05-24 9:18 ` Alex Bennée
2018-05-24 10:04 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 05/18] KVM: arm64: Convert lazy FPSIMD context switch trap to C Dave Martin
2018-05-23 19:35 ` Alex Bennée
2018-05-24 8:12 ` Christoffer Dall
2018-05-24 8:54 ` Dave Martin
2018-05-24 9:14 ` Alex Bennée [this message]
2018-05-22 16:05 ` [PATCH v10 06/18] arm64: fpsimd: Generalise context saving for non-task contexts Dave Martin
2018-05-23 20:15 ` Alex Bennée
2018-05-24 9:03 ` Dave Martin
2018-05-24 9:41 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 07/18] arm64: fpsimd: Eliminate task->mm checks Dave Martin
2018-05-23 11:48 ` Christoffer Dall
2018-05-23 13:31 ` Dave Martin
2018-05-23 14:56 ` Catalin Marinas
2018-05-23 15:03 ` Dave Martin
2018-05-23 16:42 ` Catalin Marinas
2018-05-24 8:33 ` Christoffer Dall
2018-05-24 9:16 ` Alex Bennée
2018-05-24 9:50 ` Dave Martin
2018-05-24 10:06 ` Christoffer Dall
2018-05-24 14:37 ` Dave Martin
2018-05-25 9:00 ` Christoffer Dall
2018-05-25 9:45 ` Dave Martin
2018-05-25 11:28 ` Christoffer Dall
2018-05-24 9:19 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 08/18] arm64/sve: Refactor user SVE trap maintenance for external use Dave Martin
2018-05-23 20:16 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 09/18] KVM: arm64: Repurpose vcpu_arch.debug_flags for general-purpose flags Dave Martin
2018-05-24 9:21 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 10/18] KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing Dave Martin
2018-05-24 10:09 ` Alex Bennée
2018-05-24 10:18 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 11/18] arm64/sve: Move read_zcr_features() out of cpufeature.h Dave Martin
2018-05-24 10:12 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 12/18] arm64/sve: Switch sve_pffr() argument from task to thread Dave Martin
2018-05-24 10:12 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 13/18] arm64/sve: Move sve_pffr() to fpsimd.h and make inline Dave Martin
2018-05-24 10:20 ` Alex Bennée
2018-05-24 11:22 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 14/18] KVM: arm64: Save host SVE context as appropriate Dave Martin
2018-05-23 14:59 ` Catalin Marinas
2018-05-24 9:11 ` Christoffer Dall
2018-05-24 14:49 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 15/18] KVM: arm64: Remove eager host SVE state saving Dave Martin
2018-05-24 14:54 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 16/18] KVM: arm64: Remove redundant *exit_code changes in fpsimd_guest_exit() Dave Martin
2018-05-24 9:11 ` Christoffer Dall
2018-05-24 15:02 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 17/18] KVM: arm64: Fold redundant exit code checks out of fixup_guest_exit() Dave Martin
2018-05-24 9:12 ` Christoffer Dall
2018-05-24 15:06 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 18/18] KVM: arm64: Invoke FPSIMD context switch trap from C Dave Martin
2018-05-24 15:09 ` Alex Bennée
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