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

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

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.
> 
> 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.

Thanks,
Amit D
> 
> 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
>