Re: [PATCH RFC 2/5] s390x: implement diag260

[David Hildenbrand <david@redhat.com>]

On 10.07.20 10:32, David Hildenbrand wrote:
> On 09.07.20 12:37, Cornelia Huck wrote:
>> On Wed,  8 Jul 2020 20:51:32 +0200
>> David Hildenbrand <david@redhat.com> wrote:
>>
>>> Let's implement the "storage configuration" part of diag260. This diag
>>> is found under z/VM, to indicate usable chunks of memory tot he guest OS.
>>> As I don't have access to documentation, I have no clue what the actual
>>> error cases are, and which other stuff we could eventually query using this
>>> interface. Somebody with access to documentation should fix this. This
>>> implementation seems to work with Linux guests just fine.
>>>
>>> The Linux kernel supports diag260 to query the available memory since
>>> v4.20. Older kernels / kvm-unit-tests will later fail to run in such a VM
>>> (with maxmem being defined and bigger than the memory size, e.g., "-m
>>>  2G,maxmem=4G"), just as if support for SCLP storage information is not
>>> implemented. They will fail to detect the actual initial memory size.
>>>
>>> This interface allows us to expose the maximum ramsize via sclp
>>> and the initial ramsize via diag260 - without having to mess with the
>>> memory increment size and having to align the initial memory size to it.
>>>
>>> This is a preparation for memory device support. We'll unlock the
>>> implementation with a new QEMU machine that supports memory devices.
>>>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>  target/s390x/diag.c        | 57 ++++++++++++++++++++++++++++++++++++++
>>>  target/s390x/internal.h    |  2 ++
>>>  target/s390x/kvm.c         | 11 ++++++++
>>>  target/s390x/misc_helper.c |  6 ++++
>>>  target/s390x/translate.c   |  4 +++
>>>  5 files changed, 80 insertions(+)
>>>
>>> diff --git a/target/s390x/diag.c b/target/s390x/diag.c
>>> index 1a48429564..c3b1e24b2c 100644
>>> --- a/target/s390x/diag.c
>>> +++ b/target/s390x/diag.c
>>> @@ -23,6 +23,63 @@
>>>  #include "hw/s390x/pv.h"
>>>  #include "kvm_s390x.h"
>>>  
>>> +void handle_diag_260(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
>>> +{
>>> +    MachineState *ms = MACHINE(qdev_get_machine());
>>> +    const ram_addr_t initial_ram_size = ms->ram_size;
>>> +    const uint64_t subcode = env->regs[r3];
>>> +    S390CPU *cpu = env_archcpu(env);
>>> +    ram_addr_t addr, length;
>>> +    uint64_t tmp;
>>> +
>>> +    /* TODO: Unlock with new QEMU machine. */
>>> +    if (false) {
>>> +        s390_program_interrupt(env, PGM_OPERATION, ra);
>>> +        return;
>>> +    }
>>> +
>>> +    /*
>>> +     * There also seems to be subcode "0xc", which stores the size of the
>>> +     * first chunk and the total size to r1/r2. It's only used by very old
>>> +     * Linux, so don't implement it.
>>
>> FWIW,
>> https://www-01.ibm.com/servers/resourcelink/svc0302a.nsf/pages/zVMV7R1sc246272/$file/hcpb4_v7r1.pdf
>> seems to list the available subcodes. Anything but 0xc and 0x10 is for
>> 24/31 bit only, so we can safely ignore them. Not sure what we want to
>> do with 0xc: it is supposed to "Return the highest addressable byte of
>> virtual storage in the host-primary address space, including named
>> saved systems and saved segments", so returning the end of the address
>> space should be easy enough, but not very useful.
>>
>>> +     */
>>> +    if ((r1 & 1) || subcode != 0x10) {
>>> +        s390_program_interrupt(env, PGM_SPECIFICATION, ra);
>>> +        return;
>>> +    }
>>> +    addr = env->regs[r1];
>>> +    length = env->regs[r1 + 1];
>>> +
>>> +    /* FIXME: Somebody with documentation should fix this. */
>>
>> Doc mentioned above says for specification exception:
>>
>> "For subcode X'10':
>> • Rx is not an even-numbered register.
>> • The address contained in Rx is not on a quadword boundary.
>> • The length contained in Rx+1 is not a positive multiple of 16."
>>
>>> +    if (!QEMU_IS_ALIGNED(addr, 16) || !QEMU_IS_ALIGNED(length, 16)) {
>>> +        s390_program_interrupt(env, PGM_SPECIFICATION, ra);
>>> +        return;
>>> +    }
>>> +
>>> +    /* FIXME: Somebody with documentation should fix this. */
>>> +    if (!length) {
>>
>> Probably specification exception as well?
>>
>>> +        setcc(cpu, 3);
>>> +        return;
>>> +    }
>>> +
>>> +    /* FIXME: Somebody with documentation should fix this. */
>>
>> For access exception:
>>
>> "For subcode X'10', an error occurred trying to store the extent
>> information into the guest's output area."
>>
>>> +    if (!address_space_access_valid(&address_space_memory, addr, length, true,
>>> +                                    MEMTXATTRS_UNSPECIFIED)) {
>>> +        s390_program_interrupt(env, PGM_ADDRESSING, ra);
>>> +        return;
>>> +    }
>>> +
>>> +    /* Indicate our initial memory ([0 .. ram_size - 1]) */
>>> +    tmp = cpu_to_be64(0);
>>> +    cpu_physical_memory_write(addr, &tmp, sizeof(tmp));
>>> +    tmp = cpu_to_be64(initial_ram_size - 1);
>>> +    cpu_physical_memory_write(addr + sizeof(tmp), &tmp, sizeof(tmp));
>>> +
>>> +    /* Exactly one entry was stored. */
>>> +    env->regs[r3] = 1;
>>> +    setcc(cpu, 0);
>>> +}
>>> +
>>>  int handle_diag_288(CPUS390XState *env, uint64_t r1, uint64_t r3)
>>>  {
>>>      uint64_t func = env->regs[r1];
>>
>> (...)
>>
>>> diff --git a/target/s390x/misc_helper.c b/target/s390x/misc_helper.c
>>> index 58dbc023eb..d7274eb320 100644
>>> --- a/target/s390x/misc_helper.c
>>> +++ b/target/s390x/misc_helper.c
>>> @@ -116,6 +116,12 @@ void HELPER(diag)(CPUS390XState *env, uint32_t r1, uint32_t r3, uint32_t num)
>>>      uint64_t r;
>>>  
>>>      switch (num) {
>>> +    case 0x260:
>>> +        qemu_mutex_lock_iothread();
>>> +        handle_diag_260(env, r1, r3, GETPC());
>>> +        qemu_mutex_unlock_iothread();
>>> +        r = 0;
>>> +        break;
>>>      case 0x500:
>>>          /* KVM hypercall */
>>>          qemu_mutex_lock_iothread();
>>
>> Looking at the doc referenced above, it seems that we treat every diag
>> call as privileged under tcg; but it seems that 0x44 isn't? (Unrelated
>> to your patch; maybe I'm misreading.)
> 
> That's also a BUG in kvm then?
> 
> int kvm_s390_handle_diag(struct kvm_vcpu *vcpu)
> {
> ...
> 	if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE)
> 		return kvm_s390_inject_program_int(vcpu, PGM_PRIVILEGED_OP);
> ...
> }
> 

But OTOH, it does not sound sane if user space can bypass the OS to
yield the CPU ... so this might just be a wrong documentation. All DIAGs
should be privileged IIRC.

-- 
Thanks,

David / dhildenb