Re: [PATCH v8 4/9] KVM: arm/arm64: preserve host HCR_EL2 value

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

Hi,

On 4/9/19 12:09 AM, Kristina Martsenko wrote:
> On 08/04/2019 14:05, Amit Daniel Kachhap wrote:
>> Hi James,
>>
>> On 4/6/19 4:07 PM, James Morse wrote:
>>> Hi Amit,
>>>
>>> On 02/04/2019 03:27, Amit Daniel Kachhap wrote:
>>>> From: Mark Rutland <mark.rutland@arm.com>
>>>>
>>>> When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
>>>> is a constant value. This works today, as the host HCR_EL2 value is
>>>> always the same, but this will get in the way of supporting extensions
>>>> that require HCR_EL2 bits to be set conditionally for the host.
>>>>
>>>> To allow such features to work without KVM having to explicitly handle
>>>> every possible host feature combination, this patch has KVM save/restore
>>>> for the host HCR when switching to/from a guest HCR. The saving of the
>>>> register is done once during cpu hypervisor initialization state and is
>>>> just restored after switch from guest.
>>>>
>>>> For fetching HCR_EL2 during kvm initialisation, a hyp call is made using
>>>> kvm_call_hyp and is helpful in non-VHE case.
>>>>
>>>> For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated
>>>> to toggle the TGE bit with a RMW sequence, as we already do in
>>>> __tlb_switch_to_guest_vhe().
>>>>
>>>> The value of hcr_el2 is now stored in struct kvm_cpu_context as both host
>>>> and guest can now use this field in a common way.
>>>
>>> These HCR_EL2 flags have had me confused for quite a while.
>>> I thought this was preserving the value that head.S or cpufeature.c had set, and with
>>> ptrauth we couldn't know what this register should be anymore, the host flags has to vary.
>>>
>>> Kristina's explanation of it[0], clarified things, and with a bit more digging it appears
>>> we always set API/APK, even if the hardware doesn't support the feature (as its harmless).
>>> So we don't need to vary the host flags...
>>
>> API/APK is always set for NVHE host mode.
>>>
>>> My question is, what breaks if this patch isn't merged? (the MDCR change is cleanup we can
>>> do because of this HCR change), is this HCR change just cleanup too? If so, can we merge
>>> ptrauth without either, so we only make the change when its needed? (it will cause some
>>> changes in your patch 7, but I can't see where you depend on the host flags).
>>
>> Yes you are right that this patch does not directly effect pointer authentication functionality but contains several optimizations and cleanups such as,
>>
>> * Removes assigning static flags HCR_HOST_VHE_FLAGS/HCR_HOST_NVHE_FLAGS from switch.c so switching functions now are more generic in nature.
>> * Currently the variation in hcr_el2 flags is across modes (VHE/NVHE). Any future conditional change within those modes in host HCR_EL2 may not effect code changes in switch.c
>> * Save of hcr_el2 done at hyp init time so not expensive switching wise.
>>
>> I am fine on posting it separately also.
> 
> FWIW I think it makes sense to post the HCR and MDCR patches separately
> from this series. That should make it clear that pointer auth does not
> depend on these changes, and should make it easier to evaluate the
> changes on their own.
ok.
> 
> Others' opinions are welcome as well.
> 
>>> I recall Christoffer wanting to keep the restored DAIF register value on guest-exit static
>>> to avoid extra loads/stores when we know what the value would be. I think the same logic
>>> applies here.
>> Yes the saving of host registers once was suggested by Christoffer.
> 
> I'm not familiar with this, but James may be referring to
> kvm_arm_vhe_guest_exit, which restores DAIF to a constant value. It
> seems like originally the patch saved/restored DAIF [1], but it was
> decided that a constant value was better.
Thanks for the pointer. I mis-understood it.

Thanks,
Amit D
> 
> Thanks,
> Kristina
> 
> [1] https://www.spinics.net/lists/arm-kernel/msg599798.html
> 
>>> You mentioned in the cover letter the series has some history to it!
>>>
>>>
>>> Thanks,
>>>
>>> James
>>>
>>> [0] http://lore.kernel.org/r/7ec2f950-7587-5ecd-6caa-c2fd091ad22c@arm.com
>>>
>