Re: [PATCH v5 3/4] ARM: KVM: arch_timers: Add timer world switch

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

On 14/01/13 15:21, Will Deacon wrote:
> On Tue, Jan 08, 2013 at 06:43:27PM +0000, Christoffer Dall wrote:
>> From: Marc Zyngier <marc.zyngier@arm.com>
>>
>> Do the necessary save/restore dance for the timers in the world
>> switch code. In the process, allow the guest to read the physical
>> counter, which is useful for its own clock_event_device.
> 
> [...]
> 
>> @@ -476,6 +513,7 @@ vcpu	.req	r0		@ vcpu pointer always in r0
>>   * for the host.
>>   *
>>   * Assumes vcpu pointer in vcpu reg
>> + * Clobbers r2-r4
>>   */
>>  .macro restore_timer_state
>>  	@ Disallow physical timer access for the guest
>> @@ -484,6 +522,30 @@ vcpu	.req	r0		@ vcpu pointer always in r0
>>  	orr	r2, r2, #CNTHCTL_PL1PCTEN
>>  	bic	r2, r2, #CNTHCTL_PL1PCEN
>>  	mcr	p15, 4, r2, c14, c1, 0	@ CNTHCTL
>> +
>> +#ifdef CONFIG_KVM_ARM_TIMER
>> +	ldr	r4, [vcpu, #VCPU_KVM]
>> +	ldr	r2, [r4, #KVM_TIMER_ENABLED]
>> +	cmp	r2, #0
>> +	beq	1f
>> +
>> +	ldr	r2, [r4, #KVM_TIMER_CNTVOFF]
>> +	ldr	r3, [r4, #(KVM_TIMER_CNTVOFF + 4)]
>> +	mcrr	p15, 4, r2, r3, c14	@ CNTVOFF
>> +	isb
>> +
>> +	ldr	r4, =VCPU_TIMER_CNTV_CVAL
>> +	add	vcpu, vcpu, r4
>> +	ldrd	r2, r3, [vcpu]
>> +	sub	vcpu, vcpu, r4
>> +	mcrr	p15, 3, r2, r3, c14	@ CNTV_CVAL
>> +
>> +	ldr	r2, [vcpu, #VCPU_TIMER_CNTV_CTL]
>> +	and	r2, r2, #3
>> +	mcr	p15, 0, r2, c14, c3, 1	@ CNTV_CTL
>> +	isb
> 
> How many of these isbs are actually needed, given that we're going to make
> an exception return to the guest? The last one certainly looks redundant and
> I can't see the need for ordering CNTVOFF vs CNTV_CVAL. I can see an
> argument to putting one *before* CNTV_CTL, but you don't have one there!

CNTVOFF directly influences whether or not CNTV_CVAL will trigger or
not. Maybe I'm just being paranoid and moving the isb after CNTV_CVAL is
enough.

The last one is definitively superfluous.

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