Re: [kvm-unit-tests PATCH v4 4/6] s390x: Add library functions for exiting from snippet

Janosch Frank
2024-10-18T12:53:43Z

On 10/18/24 12:56 PM, Nico Boehr wrote:
> Quoting Janosch Frank (2024-10-18 10:02:37)
> [...]
>>> +static inline uint64_t snippet_get_force_exit_value(struct vm *vm)
>>> +{
>>> + struct kvm_s390_sie_block *sblk = vm->sblk;
>>> +
>>> + assert(snippet_is_force_exit_value(vm));
>>> +
>>> + return vm->save_area.guest.grs[sblk_ip_as_diag(sblk).r_1];
>>> +}
>>
>> The cpu address parameter for 9C is 16 bit.
>> While we could make it 64 bit for snippets I don't see a reason to do
>> so. The 16 bits are enough to indicate something to the host which can
>> then go and fetch memory for more data.
>
> Mh, how exactly would you "fetch memory"? That requires knowledge on where
> things are in guest memory which can be painful to figure out from the
> host.
>
> I've found it useful to be able to pass a pointer from guest to host. Maybe
> a diag500 is the better option? gr2 contains the cookie which is a 64-bit
> value - see Linux' Documentation/virt/kvm/s390/s390-diag.rst.
>
> P.S. Did I miss the part in the docs where the 16-bit restriction of 9c is
> documented or is it missing?

For ASM snippets addresses 0x2000 to 0x4000 are a free area.
For C snippets that area is the stack.
The 16 bits should be good enough to point into that area.

If the snippet requires a lot of memory then you can use constant
addresses which are way over the snippet binary or just store a 64 bit
address in a "free" lowcore location.

As you mentioned we also have diag500 which has the drawback of
requiring to specify a couple more registers but that's not a huge
hassle. For some yet to be released PV tests I use the diag500 to
transfer 64 bits without the unshare.

If you want to write some funky code just do 4 diag9Cs to pass all 64
bits. Just don't ever show me that code :)

I've looked at the LPAR spec and that specifies bits 48-63 as the cpu
address. Send me a DM if you find a different specification.