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

[Amit Daniel Kachhap]

Hi Mark,
On 4/9/19 2:08 PM, Marc Zyngier wrote:
> On 08/04/2019 19:39, 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@xxxxxxx>
> > > > >
> > > > > 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.
> >
> > Others' opinions are welcome as well.
>
> Agreed. I'm quite eager to move forward with this series, and the least
> unrelated changes it makes, the better. Cleanups and optimizations can
> always be merged at a later time.
yes sure. I will re-spin the patch series shortly.

Thanks,
Amit D.
> Thanks,
>
> 	M.