Re: [PATCH v6 1/6] arm/arm64: KVM: Introduce armv7 fp/simd vcpu fields and helpers

[Mario Smarduch <m.smarduch@samsung.com>]

On 1/12/2016 4:57 PM, Mario Smarduch wrote:
> 
> 
> On 1/12/2016 6:12 AM, Christoffer Dall wrote:
>> On Mon, Jan 11, 2016 at 03:39:21PM -0800, Mario Smarduch wrote:
>>>
>>>
>>> On 1/10/2016 8:32 AM, Christoffer Dall wrote:
>>>> Hi Mario,
>>>>
>>>> I spotted one more potential issue...
>>>>
>>>> On Sat, Dec 26, 2015 at 01:54:55PM -0800, Mario Smarduch wrote:
>>>>> Add helper functions to enable access to fp/smid on guest entry and save host
>>>>> fpexc on vcpu put, check if fp/simd registers are dirty and add new vcpu
>>>>> fields.
>>>>>
>>>>> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
>>>>> ---
>>>>>  arch/arm/include/asm/kvm_emulate.h   | 42 ++++++++++++++++++++++++++++++++++++
>>>>>  arch/arm/include/asm/kvm_host.h      |  6 ++++++
>>>>>  arch/arm64/include/asm/kvm_emulate.h |  8 +++++++
>>>>>  3 files changed, 56 insertions(+)
>>>>>
>>>>> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
>>>>> index 3095df0..d4d9da1 100644
>>>>> --- a/arch/arm/include/asm/kvm_emulate.h
>>>>> +++ b/arch/arm/include/asm/kvm_emulate.h
>>>>> @@ -24,6 +24,8 @@
>>>>>  #include <asm/kvm_mmio.h>
>>>>>  #include <asm/kvm_arm.h>
>>>>>  #include <asm/cputype.h>
>>>>> +#include <asm/vfp.h>
>>>>> +#include "../vfp/vfpinstr.h"
>>>>>  
>>>>>  unsigned long *vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num);
>>>>>  unsigned long *vcpu_spsr(struct kvm_vcpu *vcpu);
>>>>> @@ -255,4 +257,44 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
>>>>>  	}
>>>>>  }
>>>>>  
>>>>> +#ifdef CONFIG_VFPv3
>>>>> +/* Called from vcpu_load - save fpexc and enable guest access to fp/simd unit */
>>>>> +static inline void vcpu_trap_vfp_enable(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +	u32 fpexc;
>>>>> +
>>>>> +	/* Save host fpexc, and enable guest access to fp unit */
>>>>> +	fpexc = fmrx(FPEXC);
>>>>> +	vcpu->arch.host_fpexc = fpexc;
>>>>> +	fpexc |= FPEXC_EN;
>>>>> +	fmxr(FPEXC, fpexc);
>>>>> +
>>>>> +	/* Configure HCPTR to trap on tracing and fp/simd access */
>>>>> +	vcpu->arch.hcptr = HCPTR_TTA | HCPTR_TCP(10)  | HCPTR_TCP(11);
>>>>> +}
>>>>> +
>>>>> +/* Called from vcpu_put - restore host fpexc */
>>>>> +static inline void vcpu_restore_host_fpexc(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +	fmxr(FPEXC, vcpu->arch.host_fpexc);
>>>>> +}
>>>>> +
>>>>> +/* If trap bits are reset then fp/simd registers are dirty */
>>>>> +static inline bool vcpu_vfp_isdirty(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +	return !(vcpu->arch.hcptr & (HCPTR_TCP(10) | HCPTR_TCP(11)));
>>>>> +}
>>>>> +#else
>>>>> +static inline void vcpu_trap_vfp_enable(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +	vcpu->arch.hcptr = HCPTR_TTA;
>>>>
>>>> Is it correct not to trap VFP registers when the host kernel does not
>>>> have CONFIG_VFPv3?  I think this is a change in functionality compared
>>>> to the current kernels is it not?
>>>
>>> With CPU_V7 VFPv3 gets selected, without it fp should be emulated,
>>> with exceptions taken in guest kernel. I don't see a reason why
>>> fp hcptr access should be enabled in that case.
>>>
>>
>> If you have to guests with CONFIG_VFPV3 but your host doesn't have
>> CONFIG_VFPV3, you will never context-switch the VFP registers between
>> the two VMs, and mayhem will ensue.
>>
>> Unless I'm missing something very obvious?

Did more testing on this enabling OABI_COMPAT and selecting
NWFPE/FastFPE breaks the boot. So far can't find a way to boot host
without VFP/VFPv3 enabled on ARMv7. CPU_V7 defaults to VFPv3
selection. I'm wondering if !VFPv3 path should be removed from
the patches?

- Mario

> 
> Hi Christoffer,
>    - on host I disabled VFP/VFPv3 and got a lot of Illegal instructions
>      and many other problems. Perhaps disabling for armv7 this option may need
>      re-evaluation.
>    - Enabling VFPv3 on host and running guest with no vfpv3 appears
>      to work with few glitches.
>    - and vpfv3 host/guest works just fine.
> 
> Appears disabling vfpv3 on armv7 requires another investigation (atleast
> on my end).
> 
> BTW this is on fast models.
> 
> - Mario
> 
>>
>> -Christoffer
>>
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