Re: [PATCH v6 3/6] arm64/kvm: context-switch ptrauth registers

[Amit Daniel Kachhap <amit.kachhap@arm.com>]

On 2/21/19 9:21 PM, Dave Martin wrote:
> On Thu, Feb 21, 2019 at 12:29:42PM +0000, Mark Rutland wrote:
>> On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote:
>>> From: Mark Rutland <mark.rutland@arm.com>
>>>
>>> When pointer authentication is supported, a guest may wish to use it.
>>> This patch adds the necessary KVM infrastructure for this to work, with
>>> a semi-lazy context switch of the pointer auth state.
>>>
>>> Pointer authentication feature is only enabled when VHE is built
>>> in the kernel and present into CPU implementation so only VHE code
>>> paths are modified.
>>
>> Nit: s/into/in the/
>>
>>>
>>> When we schedule a vcpu, we disable guest usage of pointer
>>> authentication instructions and accesses to the keys. While these are
>>> disabled, we avoid context-switching the keys. When we trap the guest
>>> trying to use pointer authentication functionality, we change to eagerly
>>> context-switching the keys, and enable the feature. The next time the
>>> vcpu is scheduled out/in, we start again. However the host key registers
>>> are saved in vcpu load stage as they remain constant for each vcpu
>>> schedule.
>>>
>>> Pointer authentication consists of address authentication and generic
>>> authentication, and CPUs in a system might have varied support for
>>> either. Where support for either feature is not uniform, it is hidden
>>> from guests via ID register emulation, as a result of the cpufeature
>>> framework in the host.
>>>
>>> Unfortunately, address authentication and generic authentication cannot
>>> be trapped separately, as the architecture provides a single EL2 trap
>>> covering both. If we wish to expose one without the other, we cannot
>>> prevent a (badly-written) guest from intermittently using a feature
>>> which is not uniformly supported (when scheduled on a physical CPU which
>>> supports the relevant feature). Hence, this patch expects both type of
>>> authentication to be present in a cpu.
>>>
>>> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
>>> [Only VHE, key switch from from assembly, kvm_supports_ptrauth
>>> checks, save host key in vcpu_load]
>>
>> Hmm, why do we need to do the key switch in assembly, given it's not
>> used in-kernel right now?
>>
>> Is that in preparation for in-kernel pointer auth usage? If so, please
>> call that out in the commit message.
> 
> [...]
> 
>> Huh, so we're actually doing the switch in C code...
>>
>>>   # KVM code is run at a different exception code with a different map, so
>>>   # compiler instrumentation that inserts callbacks or checks into the code may
>>> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
>>> index 675fdc1..b78cc15 100644
>>> --- a/arch/arm64/kvm/hyp/entry.S
>>> +++ b/arch/arm64/kvm/hyp/entry.S
>>> @@ -64,6 +64,12 @@ ENTRY(__guest_enter)
>>>   
>>>   	add	x18, x0, #VCPU_CONTEXT
>>>   
>>> +#ifdef	CONFIG_ARM64_PTR_AUTH
>>> +	// Prepare parameter for __ptrauth_switch_to_guest(vcpu, host, guest).
>>> +	mov	x2, x18
>>> +	bl	__ptrauth_switch_to_guest
>>> +#endif
>>
>> ... and conditionally *calling* that switch code from assembly ...
>>
>>> +
>>>   	// Restore guest regs x0-x17
>>>   	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
>>>   	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
>>> @@ -118,6 +124,17 @@ ENTRY(__guest_exit)
>>>   
>>>   	get_host_ctxt	x2, x3
>>>   
>>> +#ifdef	CONFIG_ARM64_PTR_AUTH
>>> +	// Prepare parameter for __ptrauth_switch_to_host(vcpu, guest, host).
>>> +	// Save x0, x2 which are used later in callee saved registers.
>>> +	mov	x19, x0
>>> +	mov	x20, x2
>>> +	sub	x0, x1, #VCPU_CONTEXT
>>> +	ldr	x29, [x2, #CPU_XREG_OFFSET(29)]
>>> +	bl	__ptrauth_switch_to_host
>>> +	mov	x0, x19
>>> +	mov	x2, x20
>>> +#endif
>>
>> ... which adds a load of boilerplate for no immediate gain.
>>
>> Do we really need to do this in assembly today?
> 
> If we will need to move this to assembly when we add in-kernel ptrauth
> support, it may be best to have it in assembly from the start, to reduce
> unnecessary churn.
> 
> But having a mix of C and assembly is likely to make things more
> complicated: we should go with one or the other IMHO.
ok, I will check on this.

Thanks,
Amit D
> 
> Cheers
> ---Dave
>