Re: [RFC PATCH] KVM: s390: remove extra copy of access registers into KVM_RUN

[Janosch Frank <frankja@linux.ibm.com>]

On 1/31/24 21:58, Eric Farman wrote:
> The routine ar_translation() is called by get_vcpu_asce(), which is
> called from a handful of places, such as an interception that is
> being handled during KVM_RUN processing. In that case, the access
> registers of the vcpu had been saved to a host_acrs struct and then
> the guest access registers loaded from the KVM_RUN struct prior to
> entering SIE. Saving them back to KVM_RUN at this point doesn't do
> any harm, since it will be done again at the end of the KVM_RUN
> loop when the host access registers are restored.
> 
> But that's not the only path into this code. The MEM_OP ioctl can
> be used while specifying an access register, and will arrive here.
> 
> Linux itself doesn't use the access registers for much, but it does
> squirrel the thread local storage variable into ACRs 0 and 1 in
> copy_thread() [1]. This means that the MEM_OP ioctl may copy
> non-zero access registers (the upper- and lower-halves of the TLS
> pointer) to the KVM_RUN struct, which will end up getting propogated
> to the guest once KVM_RUN ioctls occur. Since these are almost
> certainly invalid as far as an ALET goes, an ALET Specification
> Exception would be triggered if it were attempted to be used.
> 
>

Would you be able to come up with a kvm-unit-test to verify a fix and 
for regression? Hmmm, maybe a kselftest would be even easier.