Re: HCPTR cp15 writes need isb?

[Marc Zyngier <marc.zyngier@arm.com>]

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?

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