[PATCH 3/3] KVM/arm64: enable enhanced armv8 fp/simd lazy switch
Mario Smarduch
m.smarduch at samsung.com
Fri Nov 6 08:10:04 PST 2015
On 11/6/2015 3:29 AM, Christoffer Dall wrote:
> On Thu, Nov 05, 2015 at 04:57:12PM -0800, Mario Smarduch wrote:
>>
>>
>> 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 at samsung.com>
>>>> ---
>>>> 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
>> Got it.
>>>
>>>> + */
>>>> +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...
>> Since all the work is done here I placed it here.
>
> yeah, that's what I thought first, but then I thought it's typically
> easier to understand the logic in the C-code and this is technically a
> side-effect from the name of the function "kvm_restore_host_vfp_state"
> which should then be "kvm_restore_host_vfp_state_and_clear_dirty_flag"
> :)
>
Ok I'll set in C.
>>>
>>>> +
>>>> + 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?
>>
>> My understanding is you always need to set enable in fpexec32 for 32 bit guests,
>> otherwise EL1 would get the trap instead of EL2. Not sure if that's the point
>> you're making.
>>
>
> If you're expecting to trap all accesses by setting CPTR_EL2_TFP and
> you're running a 32-bit guest, you must also enable in fpexec32, because
> otherwise the trap is not taken to EL2, but to EL1 instead.
>
> Before these patches, we *always* expected to trap VFP accesses after
> entering the guest, but since that has now changed, you should only
> fiddle with fpexec32 if you are indeed trapping (first entry after
> vcpu_load) and if it's a 32-bit VM.
>
> Does that help?
Yes sure does! Puts the vcpu_load jump to EL2 vs always into perspective.
>
>>>
>>> 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.
>>
>> Sure, makes sense since this is a hot code path.
>>>
>>> Code-wise, it will be nicer to do it on vcpu_load.
>>>
>>>> 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.
>>
>> From the manuals byte stores require a word size registers on arm64/arm32.
>> The interconnect probably drops the remaining bytes.
>> Also double checked and the compiler uses same instructions.
>>
>
> duh, I didn't see the 'b' in strb - I guess I was too tired to review
> patches.
>
> -Christoffer
>
More information about the linux-arm-kernel
mailing list