Re: [PATCH v2 4/4] target/ppc: Add POWER10 exception model

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from David Gibson's message of April 16, 2021 2:28 pm:
> On Thu, Apr 15, 2021 at 03:42:27PM +1000, Nicholas Piggin wrote:
>> POWER10 adds a new bit that modifies interrupt behaviour, LPCR[HAIL],
>> and it removes support for the LPCR[AIL]=0b10 mode.
>> 
>> Reviewed-by: Cédric Le Goater <clg@kaod.org>
>> Tested-by: Cédric Le Goater <clg@kaod.org>
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>>  hw/ppc/spapr_hcall.c            |  7 ++++-
>>  target/ppc/cpu-qom.h            |  2 ++
>>  target/ppc/cpu.h                |  5 ++--
>>  target/ppc/excp_helper.c        | 51 +++++++++++++++++++++++++++++++--
>>  target/ppc/translate.c          |  3 +-
>>  target/ppc/translate_init.c.inc |  2 +-
>>  6 files changed, 62 insertions(+), 8 deletions(-)
>> 
>> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
>> index 2fbe04a689..6802cd4dc8 100644
>> --- a/hw/ppc/spapr_hcall.c
>> +++ b/hw/ppc/spapr_hcall.c
>> @@ -1396,7 +1396,12 @@ static target_ulong h_set_mode_resource_addr_trans_mode(PowerPCCPU *cpu,
>>      }
>>  
>>      if (mflags == 1) {
>> -        /* AIL=1 is reserved */
>> +        /* AIL=1 is reserved in POWER8/POWER9 */
>> +        return H_UNSUPPORTED_FLAG;
>> +    }
>> +
>> +    if (mflags == 2 && (pcc->insns_flags2 & PPC2_ISA310)) {
>> +        /* AIL=2 is also reserved in POWER10 (ISA v3.1) */
>>          return H_UNSUPPORTED_FLAG;
>>      }
>>  
>> diff --git a/target/ppc/cpu-qom.h b/target/ppc/cpu-qom.h
>> index 118baf8d41..06b6571bc9 100644
>> --- a/target/ppc/cpu-qom.h
>> +++ b/target/ppc/cpu-qom.h
>> @@ -116,6 +116,8 @@ enum powerpc_excp_t {
>>      POWERPC_EXCP_POWER8,
>>      /* POWER9 exception model           */
>>      POWERPC_EXCP_POWER9,
>> +    /* POWER10 exception model           */
>> +    POWERPC_EXCP_POWER10,
>>  };
>>  
>>  /*****************************************************************************/
>> diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
>> index 5200a16d23..9d35cdfa92 100644
>> --- a/target/ppc/cpu.h
>> +++ b/target/ppc/cpu.h
>> @@ -354,10 +354,11 @@ typedef struct ppc_v3_pate_t {
>>  #define LPCR_PECE_U_SHIFT (63 - 19)
>>  #define LPCR_PECE_U_MASK  (0x7ull << LPCR_PECE_U_SHIFT)
>>  #define LPCR_HVEE         PPC_BIT(17) /* Hypervisor Virt Exit Enable */
>> -#define LPCR_RMLS_SHIFT   (63 - 37)
>> +#define LPCR_RMLS_SHIFT   (63 - 37)   /* RMLS (removed in ISA v3.0) */
>>  #define LPCR_RMLS         (0xfull << LPCR_RMLS_SHIFT)
>> +#define LPCR_HAIL         PPC_BIT(37) /* ISA v3.1 HV AIL=3 equivalent */
>>  #define LPCR_ILE          PPC_BIT(38)
>> -#define LPCR_AIL_SHIFT    (63 - 40)      /* Alternate interrupt location */
>> +#define LPCR_AIL_SHIFT    (63 - 40)   /* Alternate interrupt location */
>>  #define LPCR_AIL          (3ull << LPCR_AIL_SHIFT)
>>  #define LPCR_UPRT         PPC_BIT(41) /* Use Process Table */
>>  #define LPCR_EVIRT        PPC_BIT(42) /* Enhanced Virtualisation */
>> diff --git a/target/ppc/excp_helper.c b/target/ppc/excp_helper.c
>> index 964a58cfdc..38a1482519 100644
>> --- a/target/ppc/excp_helper.c
>> +++ b/target/ppc/excp_helper.c
>> @@ -170,7 +170,27 @@ static int powerpc_reset_wakeup(CPUState *cs, CPUPPCState *env, int excp,
>>   * +-------------------------------------------------------+
>>   *
>>   * The difference with POWER9 being that MSR[HV] 0->1 interrupts can be sent to
>> - * the hypervisor in AIL mode if the guest is radix.
>> + * the hypervisor in AIL mode if the guest is radix. This is good for
>> + * performance but allows the guest to influence the AIL of hypervisor
>> + * interrupts using its MSR, and also the hypervisor must disallow guest
>> + * interrupts (MSR[HV] 0->0) from using AIL if the hypervisor does not want to
>> + * use AIL for its MSR[HV] 0->1 interrupts.
>> + *
>> + * POWER10 addresses those issues with a new LPCR[HAIL] bit that is applied to
>> + * interrupts that begin execution with MSR[HV]=1 (so both MSR[HV] 0->1 and
>> + * MSR[HV] 1->1).
>> + *
>> + * HAIL=1 is equivalent to AIL=3, for interrupts delivered with MSR[HV]=1.
>> + *
>> + * POWER10 behaviour is
>> + * | LPCR[AIL] | LPCR[HAIL] | MSR[IR||DR] | MSR[HV] | new MSR[HV] | AIL |
>> + * +-----------+------------+-------------+---------+-------------+-----+
>> + * | a         | h          | 00/01/10    | 0       | 0           | 0   |
>> + * | a         | h          | 11          | 0       | 0           | a   |
>> + * | a         | h          | x           | 0       | 1           | h   |
>> + * | a         | h          | 00/01/10    | 1       | 1           | 0   |
>> + * | a         | h          | 11          | 1       | 1           | h   |
>> + * +--------------------------------------------------------------------+
>>   */
>>  static inline void ppc_excp_apply_ail(PowerPCCPU *cpu, int excp_model, int excp,
>>                                        target_ulong msr,
>> @@ -210,6 +230,29 @@ static inline void ppc_excp_apply_ail(PowerPCCPU *cpu, int excp_model, int excp,
>>              /* AIL=1 is reserved */
>>              return;
>>          }
>> +
>> +    } else if (excp_model == POWERPC_EXCP_POWER10) {
>> +        if (!mmu_all_on && !hv_escalation) {
>> +            /*
>> +             * AIL works for HV interrupts even with guest MSR[IR/DR] disabled.
>> +             * Guest->guest and HV->HV interrupts do require MMU on.
>> +             */
>> +            return;
>> +        }
>> +
>> +        if (*new_msr & MSR_HVB) {
>> +            if (!(env->spr[SPR_LPCR] & LPCR_HAIL)) {
>> +                /* HV interrupts depend on LPCR[HAIL] */
>> +                return;
>> +            }
>> +            ail = 3; /* HAIL=1 gives AIL=3 behaviour for HV interrupts */
>> +        } else {
>> +            ail = (env->spr[SPR_LPCR] & LPCR_AIL) >> LPCR_AIL_SHIFT;
>> +        }
>> +        if (ail != 3) {
>> +            /* AIL=1 and AIL=2 are reserved */
>> +            return;
> 
> As with POWER9, I wonder if we should actuall filter this at LPCR
> write time and assert() here.

Could do. The processor is allowed to read back reserved bits as 0. I 
can't quite see how reserved values in multi bit fields are treated
though. Neither bits are reserved but the value 2 is. If you write 3
then 2, would the second write also clear bit 0, or would it ignore
the write and leave bit 1 set? I don't see either being explicitly
allowed so it might retain the value 2 I suspect.

> On actual hardware, what will happen if you attempt to write a bad AIL
> to the LPCR?

Good question, I don't know. We don't have an easy way to place a
interrupt handler at 0x17000 in Linux I think. I'll see if I can
get some data.

Thanks,
Nick