Re: [PATCH v2 06/13] s390x: protvirt: KVM intercept changes

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

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On 12/6/19 10:08 AM, Cornelia Huck wrote:
> On Fri, 6 Dec 2019 09:45:41 +0100
> Janosch Frank <frankja@linux.ibm.com> wrote:
> 
>> On 12/6/19 9:29 AM, Cornelia Huck wrote:
>>> On Fri, 6 Dec 2019 08:44:52 +0100
>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>   
>>>> On 12/5/19 6:46 PM, Cornelia Huck wrote:  
>>>>> On Thu, 5 Dec 2019 18:34:32 +0100
>>>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>>>     
>>>>>> On 12/5/19 6:15 PM, Cornelia Huck wrote:    
>>>>>>> On Fri, 29 Nov 2019 04:48:02 -0500
>>>>>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>>>>>       
>>>>>>>> Secure guests no longer intercept with code 4 for an instruction
>>>>>>>> interception. Instead they have codes 104 and 108 for secure
>>>>>>>> instruction interception and secure instruction notification
>>>>>>>> respectively.
>>>>>>>>
>>>>>>>> The 104 mirrors the 4 interception.
>>>>>>>>
>>>>>>>> The 108 is a notification interception to let KVM and QEMU know that
>>>>>>>> something changed and we need to update tracking information or
>>>>>>>> perform specific tasks. It's currently taken for the following
>>>>>>>> instructions:
>>>>>>>>
>>>>>>>> * stpx (To inform about the changed prefix location)
>>>>>>>> * sclp (On incorrect SCCB values, so we can inject a IRQ)
>>>>>>>> * sigp (All but "stop and store status")
>>>>>>>> * diag308 (Subcodes 0/1)
>>>>>>>>
>>>>>>>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>>>>>>>> ---
>>>>>>>>  target/s390x/kvm.c | 6 ++++++
>>>>>>>>  1 file changed, 6 insertions(+)
>>>>>>>>    
>>>>>     
>>>>>>>> @@ -1664,6 +1668,8 @@ static int handle_intercept(S390CPU *cpu)
>>>>>>>>              (long)cs->kvm_run->psw_addr);
>>>>>>>>      switch (icpt_code) {
>>>>>>>>          case ICPT_INSTRUCTION:
>>>>>>>> +        case ICPT_PV_INSTR:
>>>>>>>> +        case ICPT_PV_INSTR_NOTIFICATION:
>>>>>>>>              r = handle_instruction(cpu, run);      
>>>>>>>
>>>>>>> I'm still a bit uneasy about going through the same path for both 104
>>>>>>> and 108. How does the handler figure out whether it should emulate an
>>>>>>> instruction, or just process a notification? Is it guaranteed that a
>>>>>>> given instruction is always showing up as either a 104 or a 108, so
>>>>>>> that the handler can check the pv state?      
>>>>>>
>>>>>> diag 308 subcode 0/1 are 108, but all other subcodes are defined as a
>>>>>> 104 (if they are an exit at all)...    
>>>>>
>>>>> I think that's a reason to really split 108 from 4/104, or at least add
>>>>> an parameter...    
>>>>
>>>> And still call the diag 308 handler or have separate handlers?  
>>>
>>> I'd probably split it into a "normal" one and one for pv special
>>> handling... does that make sense?
>>>   
>> IMHO: not really
>> We still need to do ipa/ipb parsing for both paths, which will result in
>> code duplication. Looking at diag308 subcode 4, we would have a code 4
>> one which just does the device resets and reboots and one which does all
>> that, plus the teardown of the protected guest.
>>
>> I tried to inline as much as possible to have as little changes as
>> possible. Notable exception is sclp, which has more checks than
>> emulation code...
> 
> Fair enough.
> 
> But taking a step back: What's the purpose of the new exits, then?
> IIUC, we have the following cases:
> 
> - code 4: normal guest, nothing special
> - code 104: protected guest, emulate the instruction
> - code 108: protected guest, notification for the instruction
> 
> The backend code can figure out what to do simply by checking whether
> the guest is protected or not (as whatever needs to be done is simply
> determined by that anyway).
> 
> Are we overlooking something? Or is the information contained in the
> different exits simply redundant?

The difference is in the entry after the exit:

On a 104 we have a "continuation", i.e. the data that's a result of the
emulation by KVM/QEMU is used to complete the instruction. Copying the
sccb from the satellite block into guest2 memory, etc.

For a 108 we don't have any special handling (except for maybe state
checking) and just continue with the next instruction.



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