Re: [PATCH 21/27] arm64/sve: KVM: Prevent guests from using SVE

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

On 16/08/17 11:50, Dave Martin wrote:
> On Tue, Aug 15, 2017 at 05:33:15PM +0100, Marc Zyngier wrote:
>> On 09/08/17 13:05, Dave Martin wrote:
>>> Until KVM has full SVE support, guests must not be allowed to
>>> execute SVE instructions.
>>>
>>> This patch enables the necessary traps, and also ensures that the
>>> traps are disabled again on exit from the guest so that the host
>>> can still use SVE if it wants to.
>>>
>>> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
>>> ---
>>>  arch/arm64/include/asm/kvm_arm.h | 3 ++-
>>>  arch/arm64/kvm/hyp/switch.c      | 6 +++---
>>>  2 files changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
>>> index dbf0537..8a19651 100644
>>> --- a/arch/arm64/include/asm/kvm_arm.h
>>> +++ b/arch/arm64/include/asm/kvm_arm.h
>>> @@ -186,7 +186,7 @@
>>>  #define CPTR_EL2_TTA	(1 << 20)
>>>  #define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
>>>  #define CPTR_EL2_TZ	(1 << 8)
>>> -#define CPTR_EL2_DEFAULT	0x000033ff
>>> +#define CPTR_EL2_DEFAULT	(0x000033ff & ~CPTR_EL2_TZ)
>>
>> I must say I'm not overly fond of this construct. I'd rather introduce a
>> RES1 field that matches the v8.2 description, instead of this ugly
>> constant and something that clears it.
> 
> Sorry, I don't get your meaning here.  v8.2 neither immediately predates
> or postdates SVE.  

The ARMv8 ARM (DDI406B_a, D7.2.19) says otherwise. This bit is only
defined as having a possibility of being 0 on an v8.2 implementation if
SVE is implemented.

> What are you propsing?

What I'm proposing is:

#define CPTR_EL2_RES1		0x32ff
#define CPTR_EL2_DEFAULT	CPTR_EL2_RES1

and none of that pointless constant clearing.

> I guess we could just write 0x000032ff now that the only meaning the
> architecture can ever assign to bit 8 is known.
Exactly.

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