Re: [PATCH v6 2/5] selftests: kvm: s390: Add uc_skey VM test case

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

On 10/15/24 10:37 AM, Christoph Schlameuss wrote:
> Add a test case manipulating s390 storage keys from within the ucontrol
> VM.
> 
> Storage key instruction (ISKE, SSKE and RRBE) intercepts and
> Keyless-subset facility are disabled on first use, where the skeys are
> setup by KVM in non ucontrol VMs.
> 

[...]

> -/* verify SIEIC exit
> +/*
> + * Disable skey intercepts and rewind last instruction
> + * (KVM would init the skeys here)
> + */
> +static void uc_skey_enable(FIXTURE_DATA(uc_kvm) *self)
> +{
> +	struct kvm_s390_sie_block *sie_block = self->sie_block;
> +	int ilen = insn_length(sie_block->ipa >> 8);
> +	struct kvm_run *run = self->run;
> +
> +	/* disable KSS */
> +	sie_block->cpuflags &= ~CPUSTAT_KSS;
> +	/* disable skey inst interception */
> +	sie_block->ictl &= ~(ICTL_ISKE | ICTL_SSKE | ICTL_RRBE);
> +
> +	/* rewind to reexecute intercepted instruction */
> +	run->psw_addr = run->psw_addr - ilen;

There's a very important detail between KSS and the SKEY ICTLs:
KSS is (mostly) nullifying i.e. the PSW points to the instruction that 
caused the KSS exit.
ICTL intercepts are suppressing which means the PSW points after the 
instruction and hence we need to rewind the PSW if we want to re-issue 
the instruction.

Re-winding on a KSS intercept makes the guest cpu execute the 
instruction before the intercept producing instruction twice.