Re: [PATCH] target/arm: Honor HCR_EL2.TID3 trapping requirements

[Marc Zyngier <maz@kernel.org>]

On 2019-11-26 21:04, Richard Henderson wrote:
> On 11/23/19 11:56 AM, Marc Zyngier wrote:
>> HCR_EL2.TID3 mandates that access from EL1 to a long list of id
>> registers traps to EL2, and QEMU has so far ignored this 
>> requirement.
>>
>> This breaks (among other things) KVM guests that have PtrAuth 
>> enabled,
>> while the hypervisor doesn't want to expose the feature to its 
>> guest.
>> To achieve this, KVM traps the ID registers (ID_AA64ISAR1_EL1 in 
>> this
>> case), and masks out the unsupported feature.
>>
>> QEMU not honoring the trap request means that the guest observes
>> that the feature is present in the HW, starts using it, and dies
>> a horrible death when KVM injects an UNDEF, because the feature
>> *really* isn't supported.
>>
>> Do the right thing by trapping to EL2 if HCR_EL2.TID3 is set.
>>
>> Reported-by: Will Deacon <will@kernel.org>
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>> ---
>> There is a number of other trap bits missing (TID[0-2], for 
>> example),
>> but this at least gets a mainline Linux going with cpu=max.
>
> BTW, Peter, this appears to have been the bug that was causing me so 
> many
> problems on my VHE branch.  Probably *exactly* this bug wrt ptrauth,
> since that would also be included with -cpu max.
>
> I am now able to boot a kvm guest kernel to the point of the no 
> rootfs panic,
> which I wasn't before.
>
> I can only think that I mis-identified the true cause in Lyon.
>
> Anyway, thanks Marc!

Hehe, glad it fixed more than just my pet issue! :-)

         M.
-- 
Jazz is not dead. It just smells funny...