Re: [PATCH 2/3] selftests: kvm: s390: Add uc_skey VM test case

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

On 8/19/24 6:00 PM, Christoph Schlameuss wrote:
> On Fri Aug 16, 2024 at 4:36 PM CEST, Janosch Frank wrote:
>> On 8/15/24 5:45 PM, Christoph Schlameuss wrote:
[...]
>>> +TEST_F(uc_kvm, uc_skey)
>>> +{
>>> +	u64 test_vaddr = self->base_gpa + VM_MEM_SIZE - (SZ_1M / 2);
>>> +	struct kvm_sync_regs *sync_regs = &self->run->s.regs;
>>> +	struct kvm_run *run = self->run;
>>> +	u8 skeyvalue = 0x34;
>>> +
>>> +	/* copy test_skey_asm to code_hva / code_gpa */
>>> +	TH_LOG("copy code %p to vm mapped memory %p / %p",
>>> +	       &test_skey_asm, (void *)self->code_hva, (void *)self->code_gpa);
>>> +	memcpy((void *)self->code_hva, &test_skey_asm, PAGE_SIZE);
>>> +
>>> +	/* set register content for test_skey_asm to access not mapped memory */
>>> +	sync_regs->gprs[1] = skeyvalue;
>>> +	sync_regs->gprs[5] = self->base_gpa;
>>> +	sync_regs->gprs[6] = test_vaddr;
>>> +	run->kvm_dirty_regs |= KVM_SYNC_GPRS;
>>> +
>>> +	self->sie_block->ictl |= ICTL_OPEREXC | ICTL_PINT;
>>> +	self->sie_block->cpuflags &= ~CPUSTAT_KSS;
>>
>> So you don't want KVM to initialize skeys?
>> Or am I missing a ucontrol skey interaction?
>>
>> What about the ICTLs if KSS is not available on the machine?
> 
> This is explicitly disabling KSS, not enabling it.
> Doing that explicitly might not strictly be necessary but I thought this does
> provide some clarity about the state.
> 

The 3 skey ICTLs and KSS are used by KVM to get an intercept on the 
first skey instruction that the guest issues. KVM uses that intercept to 
initialize the keys and setup skey handling since it's an edge case 
because Linux doesn't use skeys.

If KSS is available KVM will not set the skey ICTLs but KSS is a 
"recent" addition (my guess would be ~z13). So if you want to disable 
skey intercepts regardless of the machine you need to clear all 4 bits.

Are you sure that disabling KSS makes sense and does what you think it does?