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

[David Hildenbrand]

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.

Thanks for the link to the documentation! Either my google search skills
are bad or that stuff is just hard to find :) I'll have a look and see
how to make sense of 0xc. Smells like "maxram_size - 1" indeed.

> 
>> +     */
>> +    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?

Yeah I'll add "|| !length" above.

> 
>> +        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."
> 

Okay, looks good then!

>> +    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.)

Interesting. Adding in onto my todo list.

Thanks again!

-- 
Thanks,

David / dhildenb