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.
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@xxxxxxx
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