Subject: [PATCH] arm: KVM: force execution of HCPTR access on VM exit

From: [email-masked] (Marc Zyngier)
Date: Wed, 08 Apr 2015 18:16:50 +0100

On 08/04/15 12:40, Christoffer Dall wrote:
> On Mon, Mar 16, 2015 at 10:59:43AM +0000, Marc Zyngier wrote:
>> On VM entry, we disable access to the VFP registers in order to
>> perform a lazy save/restore of these registers.
>>
>> On VM exit, we restore access, test if we did enable them before,
>> and save/restore the guest/host registers if necessary. In this
>> sequence, the FPEXC register is always accessed, irrespective
>> of the trapping configuration.
>>
>> If the guest didn't touch the VFP registers, then the HCPTR access
>> has now enabled such access, but we're missing a barrier to ensure
>> architectural execution of the new HCPTR configuration. If the HCPTR
>> access has been delayed/reordered, the subsequent access to FPEXC
>> will cause a trap, which we aren't prepared to handle at all.
>>
>> The fix is to introduce a barrier that only takes place if the
>> guest hasn't accessed its view of the VFP registers, making
>> the access to FPEXC safe.
>>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm/kvm/interrupts.S | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
>> index 79caf79..3ac7aca 100644
>> --- a/arch/arm/kvm/interrupts.S
>> +++ b/arch/arm/kvm/interrupts.S
>> @@ -175,10 +175,13 @@ __kvm_vcpu_return:
>>  #ifdef CONFIG_VFPv3
>>  	@ Save floating point registers we if let guest use them.
>>  	tst	r2, #(HCPTR_TCP(10) | HCPTR_TCP(11))
>> -	bne	after_vfp_restore
>> +	beq	1f
>> +
>> +	isb	@ Force execution of HCPTR if we've just reenabled VFP access
>> +	b	after_vfp_restore
>>  
>>  	@ Switch VFP/NEON hardware state to the host's
>> -	add	r7, vcpu, #VCPU_VFP_GUEST
>> +1:	add	r7, vcpu, #VCPU_VFP_GUEST
>>  	store_vfp_state r7
>>  	add	r7, vcpu, #VCPU_VFP_HOST
>>  	ldr	r7, [r7]
>> -- 
>> 2.1.4
>>
> 
> Reviewed-by: Christoffer Dall <[email-masked]>

Thanks (I must admit I completely forgot the existence of this patch...).

I guess we can queue that for -rc1, with a Cc to stable...

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