Re: [Qemu-devel] [Qemu-ppc] [PATCH 4/5] target-ppc: Handle ibm, nmi-register RTAS call

[Alexander Graf <agraf@suse.de>]

On 28.08.14 19:42, Aravinda Prasad wrote:
> 
> 
> On Thursday 28 August 2014 02:07 PM, Alexander Graf wrote:
>>
>>
>> On 28.08.14 08:38, Aravinda Prasad wrote:
>>>
>>>
>>> On Wednesday 27 August 2014 04:07 PM, Alexander Graf wrote:
>>>>
>>>>
>>>> On 25.08.14 15:45, Aravinda Prasad wrote:
>>>>> This patch adds FWNMI support in qemu for powerKVM
>>>>> guests by handling the ibm,nmi-register rtas call.
>>>>> Whenever OS issues ibm,nmi-register RTAS call, the
>>>>> machine check notification address is saved and the
>>>>> machine check interrupt vector 0x200 is patched to
>>>>> issue a private hcall.
>>>>>
>>>>> Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
>>>>> ---
>>>>>  hw/ppc/spapr_rtas.c    |   91 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>  include/hw/ppc/spapr.h |    8 ++++
>>>>>  2 files changed, 98 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
>>>>> index 02ddbf9..1135d2b 100644
>>>>> --- a/hw/ppc/spapr_rtas.c
>>>>> +++ b/hw/ppc/spapr_rtas.c
>>>>> @@ -277,6 +277,91 @@ static void rtas_ibm_set_system_parameter(PowerPCCPU *cpu,
>>>>>      rtas_st(rets, 0, ret);
>>>>>  }
>>>>>  
>>>>> +static void rtas_ibm_nmi_register(PowerPCCPU *cpu,
>>>>> +                                  sPAPREnvironment *spapr,
>>>>> +                                  uint32_t token, uint32_t nargs,
>>>>> +                                  target_ulong args,
>>>>> +                                  uint32_t nret, target_ulong rets)
>>>>> +{
>>>>> +    int i;
>>>>> +    uint32_t branch_inst = 0x48000002;
>>>>> +    target_ulong guest_machine_check_addr;
>>>>> +    PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
>>>>> +    /*
>>>>> +     * Trampoline saves r3 in sprg2 and issues private hcall
>>>>> +     * to request qemu to build error log. QEMU builds the
>>>>> +     * error log, copies to rtas-blob and returns the address.
>>>>> +     * The initial 16 bytes in rtas-blob consists of saved srr0
>>>>> +     * and srr1 which we restore and pass on the actual error
>>>>> +     * log address to OS handled mcachine check notification
>>>>> +     * routine
>>>>> +     */
>>>>> +    uint32_t trampoline[] = {
>>>>> +        0x7c7243a6,    /* mtspr   SPRN_SPRG2,r3 */
>>>>> +        0x38600000,    /* li      r3,0   */
>>>>> +        /* 0xf004 is the KVMPPC_H_REPORT_ERR private HCALL */
>>>>> +        0x6063f004,    /* ori     r3,r3,f004  */
>>>>> +        /* Issue H_CALL */
>>>>> +        0x44000022,    /*  sc      1     */
>>>>
>>>> So up to here we're saving r3 in SPRG2 (how do we know that we can
>>>> clobber it?) and call our special hypercall.
>>>>
>>>> But what does all the cruft below here do?
>>>
>>> The saved r3 in SPRG2 is consumed in KVMPPC_H_REPORT_ERR hcall, hence we
>>> can clobber SPRG2 after hcall returns. I have included a comment in
>>> patch 3/5 while building error log. I think better I add one here as well.
>>>
>>>>
>>>>> +        0x7c9243a6,    /* mtspr r4 sprg2 */
>>>>
>>>> Apart from th fact that your order is wrong, this destroys the value of
>>>> r3 that we saved above again.
>>>
>>> SPRG2 is saved inside hcall and hence we don't need SPRG2 further after
>>> KVMPPC_H_REPORT_ERR hcall returns.
>>>
>>>>
>>>>> +        0xe8830000,    /* ld r4, 0(r3) */
>>>>> +        0x7c9a03a6,    /* mtspr r4, srr0 */
>>>>> +        0xe8830008,    /* ld r4, 8(r3) */
>>>>> +        0x7c9b03a6,    /* mtspr r4, srr1 */
>>>>
>>>> Can't we just set srr0 and srr1 directly?
>>>
>>> I checked for instructions in ISA which set srr0/1 directly given an
>>> address, but could not find any such instructions.
>>
>> I mean from QEMU :).
> 
> srr0 and srr1, which are properly set when 0x200 is invoked, are
> clobbered when we return from KVMPPC_H_REPORT_ERR hcall. I think they
> are modified before issuing rfid (I can see them getting clobbered from
> QEMU monitor). However when we jump to OS registered machine check
> routine srr0 and srr1 should reflect the value they had when 0x200 was
> invoked.
> 
> Hence srr0 and srr1 are saved in hcall and restored when we return from
> hcall. Also we don't have enough scratch registers available to save
> these before invoking hcall from 0x200.
> 
> Or am I missing other ways to do this from QEMU?

If you just do

cpu_synchronize_state() and then change env->spr[SPRN_SRR0/1] inside
your hypercall handler that should also change the value when you return
from the hcall.


Alex