Subject: [PATCH v2 04/21] arm64: KVM: Implement vgic-v3 save/restore

From: [email-masked] (Christoffer Dall)
Date: Tue, 1 Dec 2015 13:24:18 +0100

On Tue, Dec 01, 2015 at 11:57:16AM +0000, Marc Zyngier wrote:
> On 01/12/15 11:50, Christoffer Dall wrote:
> > On Tue, Dec 01, 2015 at 12:44:26PM +0100, Christoffer Dall wrote:
> >> On Tue, Dec 01, 2015 at 11:32:20AM +0000, Marc Zyngier wrote:
> >>> On 30/11/15 19:50, Christoffer Dall wrote:
> >>>> On Fri, Nov 27, 2015 at 06:49:58PM +0000, Marc Zyngier wrote:
> >>>>> Implement the vgic-v3 save restore as a direct translation of
> >>>>> the assembly code version.
> >>>>>
> >>>>> Signed-off-by: Marc Zyngier <[email-masked]>
> >>>>> ---
> >>>>>  arch/arm64/kvm/hyp/Makefile     |   1 +
> >>>>>  arch/arm64/kvm/hyp/hyp.h        |   3 +
> >>>>>  arch/arm64/kvm/hyp/vgic-v3-sr.c | 222 ++++++++++++++++++++++++++++++++++++++++
> >>>>>  3 files changed, 226 insertions(+)
> >>>>>  create mode 100644 arch/arm64/kvm/hyp/vgic-v3-sr.c
> >>>>>
> >>>>> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> >>>>> index d8d5968..d1e38ce 100644
> >>>>> --- a/arch/arm64/kvm/hyp/Makefile
> >>>>> +++ b/arch/arm64/kvm/hyp/Makefile
> >>>>> @@ -3,3 +3,4 @@
> >>>>>  #
> >>>>>  
> >>>>>  obj-$(CONFIG_KVM_ARM_HOST) += vgic-v2-sr.o
> >>>>> +obj-$(CONFIG_KVM_ARM_HOST) += vgic-v3-sr.o
> >>>>> diff --git a/arch/arm64/kvm/hyp/hyp.h b/arch/arm64/kvm/hyp/hyp.h
> >>>>> index 78f25c4..a31cb6e 100644
> >>>>> --- a/arch/arm64/kvm/hyp/hyp.h
> >>>>> +++ b/arch/arm64/kvm/hyp/hyp.h
> >>>>> @@ -30,5 +30,8 @@
> >>>>>  void __vgic_v2_save_state(struct kvm_vcpu *vcpu);
> >>>>>  void __vgic_v2_restore_state(struct kvm_vcpu *vcpu);
> >>>>>  
> >>>>> +void __vgic_v3_save_state(struct kvm_vcpu *vcpu);
> >>>>> +void __vgic_v3_restore_state(struct kvm_vcpu *vcpu);
> >>>>> +
> >>>>>  #endif /* __ARM64_KVM_HYP_H__ */
> >>>>>  
> >>>>> diff --git a/arch/arm64/kvm/hyp/vgic-v3-sr.c b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> >>>>> new file mode 100644
> >>>>> index 0000000..b490db5
> >>>>> --- /dev/null
> >>>>> +++ b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> >>>>> @@ -0,0 +1,222 @@
> >>>>> +/*
> >>>>> + * Copyright (C) 2012-2015 - ARM Ltd
> >>>>> + * Author: Marc Zyngier <[email-masked]>
> >>>>> + *
> >>>>> + * This program is free software; you can redistribute it and/or modify
> >>>>> + * it under the terms of the GNU General Public License version 2 as
> >>>>> + * published by the Free Software Foundation.
> >>>>> + *
> >>>>> + * This program is distributed in the hope that it will be useful,
> >>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >>>>> + * GNU General Public License for more details.
> >>>>> + *
> >>>>> + * You should have received a copy of the GNU General Public License
> >>>>> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> >>>>> + */
> >>>>> +
> >>>>> +#include <linux/compiler.h>
> >>>>> +#include <linux/irqchip/arm-gic-v3.h>
> >>>>> +#include <linux/kvm_host.h>
> >>>>> +
> >>>>> +#include <asm/kvm_mmu.h>
> >>>>> +
> >>>>> +#include "hyp.h"
> >>>>> +
> >>>>> +/*
> >>>>> + * We store LRs in reverse order to let the CPU deal with streaming
> >>>>> + * access. Use this macro to make it look saner...
> >>>>> + */
> >>>>> +#define LR_OFFSET(n)	(15 - n)
> >>>>> +
> >>>>> +#define read_gicreg(r)							\
> >>>>> +	({								\
> >>>>> +		u64 reg;						\
> >>>>> +		asm volatile("mrs_s %0, " __stringify(r) : "=r" (reg));	\
> >>>>> +		reg;							\
> >>>>> +	})
> >>>>> +
> >>>>> +#define write_gicreg(v,r)						\
> >>>>> +	do {								\
> >>>>> +		u64 __val = (v);					\
> >>>>> +		asm volatile("msr_s " __stringify(r) ", %0" : : "r" (__val));\
> >>>>> +	} while (0)
> >>>>
> >>>> remind me what the msr_s and mrs_s do compared to msr and mrs?
> >>>
> >>> They do the same job, only for the system registers which are not in the
> >>> original ARMv8 architecture spec, and most likely not implemented by
> >>> old(er) compilers.
> >>>
> >>>> are these the reason why we need separate macros to access the gic
> >>>> registers compared to 'normal' sysregs?
> >>>
> >>> Indeed.
> >>>
> >>>>> +
> >>>>> +/* vcpu is already in the HYP VA space */
> >>>>> +void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
> >>>>> +{
> >>>>> +	struct vgic_v3_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v3;
> >>>>> +	u64 val;
> >>>>> +	u32 nr_lr, nr_pri;
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * Make sure stores to the GIC via the memory mapped interface
> >>>>> +	 * are now visible to the system register interface.
> >>>>> +	 */
> >>>>> +	dsb(st);
> >>>>> +
> >>>>> +	cpu_if->vgic_vmcr  = read_gicreg(ICH_VMCR_EL2);
> >>>>> +	cpu_if->vgic_misr  = read_gicreg(ICH_MISR_EL2);
> >>>>> +	cpu_if->vgic_eisr  = read_gicreg(ICH_EISR_EL2);
> >>>>> +	cpu_if->vgic_elrsr = read_gicreg(ICH_ELSR_EL2);
> >>>>> +
> >>>>> +	write_gicreg(0, ICH_HCR_EL2);
> >>>>> +	val = read_gicreg(ICH_VTR_EL2);
> >>>>> +	nr_lr = val & 0xf;
> >>>>
> >>>> this is not technically nr_lr, it's max_lr or max_lr_idx or something
> >>>> like that.
> >>>
> >>> Let's go for max_lr_idx  then.
> >>>
> >>>>> +	nr_pri = ((u32)val >> 29) + 1;
> >>>>
> >>>> nit: nr_pri_bits
> >>>>
> >>>>> +
> >>>>> +	switch (nr_lr) {
> >>>>> +	case 15:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(15)] = read_gicreg(ICH_LR15_EL2);
> >>>>> +	case 14:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(14)] = read_gicreg(ICH_LR14_EL2);
> >>>>> +	case 13:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(13)] = read_gicreg(ICH_LR13_EL2);
> >>>>> +	case 12:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(12)] = read_gicreg(ICH_LR12_EL2);
> >>>>> +	case 11:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(11)] = read_gicreg(ICH_LR11_EL2);
> >>>>> +	case 10:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(10)] = read_gicreg(ICH_LR10_EL2);
> >>>>> +	case 9:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(9)] = read_gicreg(ICH_LR9_EL2);
> >>>>> +	case 8:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(8)] = read_gicreg(ICH_LR8_EL2);
> >>>>> +	case 7:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(7)] = read_gicreg(ICH_LR7_EL2);
> >>>>> +	case 6:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(6)] = read_gicreg(ICH_LR6_EL2);
> >>>>> +	case 5:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(5)] = read_gicreg(ICH_LR5_EL2);
> >>>>> +	case 4:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(4)] = read_gicreg(ICH_LR4_EL2);
> >>>>> +	case 3:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(3)] = read_gicreg(ICH_LR3_EL2);
> >>>>> +	case 2:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(2)] = read_gicreg(ICH_LR2_EL2);
> >>>>> +	case 1:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(1)] = read_gicreg(ICH_LR1_EL2);
> >>>>> +	case 0:
> >>>>> +		cpu_if->vgic_lr[LR_OFFSET(0)] = read_gicreg(ICH_LR0_EL2);
> >>>>
> >>>> I don't understand this; LR_OFFSET(0) == (15 - 0) == 15, so
> >>>>
> >>>> cpu_if->vgic_lr[15] = read_gicreg(ICH_LR0_EL2) ?
> >>>
> >>> Just like in the assembly version. We store the LRs in the order we read
> >>> them so that we don't confuse the CPU by writing backward (believe it or
> >>> not, CPUs do get horribly confused if you do that).
> >>
> >> but aren't we storing the wrong register to the wrong index in the
> >> array?
> >>
> >> Do we really access cpu_if->vgic_lr[15..12] in the C-code if the system
> >> only has 4 LRs?
> >>
> > ok, I looked at the code myself (not sure why I didn't do that in the
> > first place) and indeed we use a different but with similar results
> > macro to access the array from the C code.
> > 
> > This is just insane to me, and we don't have a comment on the data
> > structure saying "this is not stored the way you'd think it is".
> > 
> > Why can't we just do:
> > 
> > cpu_if->vgic_lr[3] = read_gicreg(ICH_LR3_EL2);
> > cpu_if->vgic_lr[2] = read_gicreg(ICH_LR2_EL2);
> > cpu_if->vgic_lr[1] = read_gicreg(ICH_LR1_EL2);
> > cpu_if->vgic_lr[0] = read_gicreg(ICH_LR0_EL2);
> > 
> > ?
> 
> Because you're *really* killing performance by doing what are
> essentially streaming read/writes in the opposite direction. CPU
> prefetchers only work in one direction (incrementing the address). Doing
> it backwards breaks it.
> 
hmm, and what are prefetching with the stores here?

Did anyone actually measure this or is it theoretically really slow?

Anyway, for the purposes of rewriting the world-switch in C, this looks
fine.  I hope we can come up with something less convoluted some time,
perhaps at least using the same macro for LR_INDEX and making the
comments more clear.

-Christoffer