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

From: [email-masked] (Marc Zyngier)
Date: Tue, 01 Dec 2015 11:57:16 +0000

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.

	M.
-- 
Jazz is not dead. It just smells funny...