On 11/5/2015 7:02 AM, Christoffer Dall wrote:
> On Fri, Oct 30, 2015 at 02:56:33PM -0700, Mario Smarduch wrote:
>> This patch enables arm64 lazy fp/simd switch, similar to arm described in
>> second patch. Change from previous version - restore function is moved to
>> host.
>>
>> Signed-off-by: Mario Smarduch <m.smarduch@xxxxxxxxxxx>
>> ---
>> arch/arm64/include/asm/kvm_host.h | 2 +-
>> arch/arm64/kernel/asm-offsets.c | 1 +
>> arch/arm64/kvm/hyp.S | 37 +++++++++++++++++++++++++++++++------
>> 3 files changed, 33 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 26a2347..dcecf92 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -251,11 +251,11 @@ static inline void kvm_arch_hardware_unsetup(void) {}
>> static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>> -static inline void kvm_restore_host_vfp_state(struct kvm_vcpu *vcpu) {}
>>
>> void kvm_arm_init_debug(void);
>> void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
>> void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
>> void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
>> +void kvm_restore_host_vfp_state(struct kvm_vcpu *vcpu);
>>
>> #endif /* __ARM64_KVM_HOST_H__ */
>> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
>> index 8d89cf8..c9c5242 100644
>> --- a/arch/arm64/kernel/asm-offsets.c
>> +++ b/arch/arm64/kernel/asm-offsets.c
>> @@ -124,6 +124,7 @@ int main(void)
>> DEFINE(VCPU_HCR_EL2, offsetof(struct kvm_vcpu, arch.hcr_el2));
>> DEFINE(VCPU_MDCR_EL2, offsetof(struct kvm_vcpu, arch.mdcr_el2));
>> DEFINE(VCPU_IRQ_LINES, offsetof(struct kvm_vcpu, arch.irq_lines));
>> + DEFINE(VCPU_VFP_DIRTY, offsetof(struct kvm_vcpu, arch.vfp_dirty));
>> DEFINE(VCPU_HOST_CONTEXT, offsetof(struct kvm_vcpu, arch.host_cpu_context));
>> DEFINE(VCPU_HOST_DEBUG_STATE, offsetof(struct kvm_vcpu, arch.host_debug_state));
>> DEFINE(VCPU_TIMER_CNTV_CTL, offsetof(struct kvm_vcpu, arch.timer_cpu.cntv_ctl));
>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> index e583613..ed2c4cf 100644
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -36,6 +36,28 @@
>> #define CPU_SYSREG_OFFSET(x) (CPU_SYSREGS + 8*x)
>>
>> .text
>> +
>> +/**
>> + * void kvm_restore_host_vfp_state(struct vcpu *vcpu) - Executes lazy
>> + * fp/simd switch, saves the guest, restores host. Called from host
>> + * mode, placed outside of hyp section.
>
> same comments on style as previous patch
>
>> + */
>> +ENTRY(kvm_restore_host_vfp_state)
>> + push xzr, lr
>> +
>> + add x2, x0, #VCPU_CONTEXT
>> + mov w3, #0
>> + strb w3, [x0, #VCPU_VFP_DIRTY]
>
> I've been discussing with myself if it would make more sense to clear
> the dirty flag in the C-code...
>
>> +
>> + bl __save_fpsimd
>> +
>> + ldr x2, [x0, #VCPU_HOST_CONTEXT]
>> + bl __restore_fpsimd
>> +
>> + pop xzr, lr
>> + ret
>> +ENDPROC(kvm_restore_host_vfp_state)
>> +
>> .pushsection .hyp.text, "ax"
>> .align PAGE_SHIFT
>>
>> @@ -482,7 +504,11 @@
>> 99:
>> msr hcr_el2, x2
>> mov x2, #CPTR_EL2_TTA
>> +
>> + ldrb w3, [x0, #VCPU_VFP_DIRTY]
>> + tbnz w3, #0, 98f
>> orr x2, x2, #CPTR_EL2_TFP
>> +98:
>
> mmm, don't you need to only set the fpexc32 when you're actually going
> to trap the guest accesses?
>
> also, you can consider only setting this in vcpu_load (jumping quickly
> to EL2 to do so) if we're running a 32-bit guest. Probably worth
> measuring the difference between the extra EL2 jump on vcpu_load
> compared to hitting this register on every entry/exit.
>
> Code-wise, it will be nicer to do it on vcpu_load.
Hi Christoffer, Marc -
just want to run this by you, I ran a test with typical number of
fp threads and couple lmbench benchmarks the stride and bandwidth ones. The
ratio of exits to vcpu puts is high 50:1 or so. But of course that's subject
to the loads you run.
I substituted:
tbnz x2, #HCR_RW_SHIFT, 99f
mov x3, #(1 << 30)
msr fpexc32_el2, x3
isb
with vcpu_load hyp call and check for 32 bit guest in C
mov x1, #(1 << 30)
msr fpexc32_el2, x3
ret
And then
skip_fpsimd_state x8, 2f
mrs x6, fpexec_el2
str x6, [x3, #16]
with vcpu_put hyp call with check for 32 bit guest in C this is called
substantially less often then vcpu_load since fp/simd registers are not
always dirty on vcpu_put
kern_hyp_va x0
add x2, x0, #VCPU_CONTEXT
mrs x1, fpexec32_el2
str x1, [x2, #CPU_SYSREG_OFFSET(FPEXC32_EL2)]
ret
Of course each hyp call has additional overhead, at a high exit to
vcpu_put ratio hyp call appears better. But all this is very
highly dependent on exit rate and fp/simd usage. IMO hyp call
works better under extreme loads should be pretty close
for general loads.
Any thoughts?
- Mario
>
>> msr cptr_el2, x2
>>
>> mov x2, #(1 << 15) // Trap CP15 Cr=15
>> @@ -669,14 +695,12 @@ __restore_debug:
>> ret
>>
>> __save_fpsimd:
>> - skip_fpsimd_state x3, 1f
>> save_fpsimd
>> -1: ret
>> + ret
>>
>> __restore_fpsimd:
>> - skip_fpsimd_state x3, 1f
>> restore_fpsimd
>> -1: ret
>> + ret
>>
>> switch_to_guest_fpsimd:
>> push x4, lr
>> @@ -688,6 +712,9 @@ switch_to_guest_fpsimd:
>>
>> mrs x0, tpidr_el2
>>
>> + mov w2, #1
>> + strb w2, [x0, #VCPU_VFP_DIRTY]
>
> hmm, just noticing this. Are you not writing a 32-bit value to a
> potentially 8-bit field (ignoring padding in the struct), as the dirty
> flag is declared a bool.
>
> Are you also doing this on the 32-bit side?
>
>> +
>> ldr x2, [x0, #VCPU_HOST_CONTEXT]
>> kern_hyp_va x2
>> bl __save_fpsimd
>> @@ -763,7 +790,6 @@ __kvm_vcpu_return:
>> add x2, x0, #VCPU_CONTEXT
>>
>> save_guest_regs
>> - bl __save_fpsimd
>> bl __save_sysregs
>>
>> skip_debug_state x3, 1f
>> @@ -784,7 +810,6 @@ __kvm_vcpu_return:
>> kern_hyp_va x2
>>
>> bl __restore_sysregs
>> - bl __restore_fpsimd
>> /* Clear FPSIMD and Trace trapping */
>> msr cptr_el2, xzr
>>
>> --
>> 1.9.1
>>
>
> Thanks,
> -Christoffer
>
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