Re: HCPTR cp15 writes need isb?

[Vikram Sethi <vikrams@codeaurora.org>]

On 06/16/15 07:47, Marc Zyngier wrote:
> On 16/06/15 13:30, Vikram Sethi wrote:
>> On 06/16/15 01:46, Marc Zyngier wrote:
>>> On Tue, 16 Jun 2015 02:34:23 +0100
>>> Vikram Sethi <vikrams@codeaurora.org> wrote:
>>>
>>> Hi Vikram,
>>>
>>>> Hi Marc, Christoffer, Catalin, Will,
>>>>
>>>> I'm seeing an issue with KVM HCPTR (cp15) writes on guest entry/exit
>>>> on one of Qualcomm's CPU cores in AArch32 host and AArch32 guest
>>>> mode. Our CPU architects believe that HCPTR cp15 writes are context
>>>> changing and require an isb. With an isb in set_hcptr macro in
>>>> arch/arm/kvm/interrupts_head.S I am able to boot the Aarch32 guest,
>>>> but without it, I see strange crashes to hyp_undef or hyp_pabt.
>>> [...]
>>>
>>> Can you look at the following patch (queued for 4.2)?
>>>
>>> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/330955.html
>>>
>>> Please let me know if this solves the issue you are seeing.
>> Don't we have the same issue the first time guest touches FP and
>> traps i.e in switch_to_guest_vfp where we turn on floating point
>> access in HCPTR and immediately access FPEXC in store_vfp_state
>> without a isb?
> Good point, looks like my initial fix is incomplete. I'll repost a more
> complete fix but in the meantime, does adding the following work for you?
Yes, the additional isb in switch_to_guest_vfp along with your original patch works for me.
When you refactor the original patch will it be cleaner to handle the isb in the set_hcptr macro whenever it is changed to not trap VFP access?

>
> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> index 3ac7aca..5b30047 100644
> --- a/arch/arm/kvm/interrupts.S
> +++ b/arch/arm/kvm/interrupts.S
> @@ -487,6 +487,7 @@ switch_to_guest_vfp:
>
>  	@ NEON/VFP used.  Turn on VFP access.
>  	set_hcptr vmexit, (HCPTR_TCP(10) | HCPTR_TCP(11))
> +	isb	@ Force execution of HCPTR as we've just reenabled VFP access
>
>  	@ Switch VFP/NEON hardware state to the guest's
>  	add	r7, r0, #VCPU_VFP_HOST
>
> Thanks,
>
> 	M.


-- 
Vikram Sethi
Qualcomm Technologies Inc, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project