[1/2] target/riscv: prioritize pmp errors in raise_mmu_exception()

From: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

raise_mmu_exception(), as is today, is prioritizing guest page faults by
checking first if virt_enabled && !first_stage, and then considering the
regular inst/load/store faults.

There's no mention in the spec about guest page fault being a higher
priority that PMP faults. In fact, privileged spec section 3.7.1 says:

"Attempting to fetch an instruction from a PMP region that does not have
execute permissions raises an instruction access-fault exception.
Attempting to execute a load or load-reserved instruction which accesses
a physical address within a PMP region without read permissions raises a
load access-fault exception. Attempting to execute a store,
store-conditional, or AMO instruction which accesses a physical address
within a PMP region without write permissions raises a store
access-fault exception."

So, in fact, we're doing it wrong - PMP faults should always be thrown,
regardless of also being a first or second stage fault.

The way riscv_cpu_tlb_fill() and get_physical_address() work is
adequate: a TRANSLATE_PMP_FAIL error is immediately reported and
reflected in the 'pmp_violation' flag. What we need is to change
raise_mmu_exception() to prioritize it.

Reported-by: Joseph Chan <jchan@ventanamicro.com>
Fixes: 82d53adfbb ("target/riscv/cpu_helper.c: Invalid exception on MMU translation stage")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/cpu_helper.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

FYI

Priv-v1.12/riscv-privileged-20211203.pdf
<https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf>
defines exception priorities on
Page 40, Table 3.7
Page 130, Table 8.7

There is a sentence under Table 3.7:
"When a virtual address is translated into a physical address, the address
translation algorithm
determines what specific exception may be raised."


The spec does not insist any implementation to report Exception Code 12
over 1; 13,15 over 5, 7. On the other hand, the phrases "During instruction
address translation:" and "With physical address for instruction:" gives me
the impression that when the implementation can distinguish between these
situations, then reporting 12 , 13, 15 instead of 1, 5, 7 will provide a
fine-grained reason for why things were broken.

Regards,
Joseph Chan


On Sat, Apr 13, 2024 at 3:59 AM Alexei Filippov <
alexei.filippov@syntacore.com> wrote:

> From: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
>
> raise_mmu_exception(), as is today, is prioritizing guest page faults by
> checking first if virt_enabled && !first_stage, and then considering the
> regular inst/load/store faults.
>
> There's no mention in the spec about guest page fault being a higher
> priority that PMP faults. In fact, privileged spec section 3.7.1 says:
>
> "Attempting to fetch an instruction from a PMP region that does not have
> execute permissions raises an instruction access-fault exception.
> Attempting to execute a load or load-reserved instruction which accesses
> a physical address within a PMP region without read permissions raises a
> load access-fault exception. Attempting to execute a store,
> store-conditional, or AMO instruction which accesses a physical address
> within a PMP region without write permissions raises a store
> access-fault exception."
>
> So, in fact, we're doing it wrong - PMP faults should always be thrown,
> regardless of also being a first or second stage fault.
>
> The way riscv_cpu_tlb_fill() and get_physical_address() work is
> adequate: a TRANSLATE_PMP_FAIL error is immediately reported and
> reflected in the 'pmp_violation' flag. What we need is to change
> raise_mmu_exception() to prioritize it.
>
> Reported-by: Joseph Chan <jchan@ventanamicro.com>
> Fixes: 82d53adfbb ("target/riscv/cpu_helper.c: Invalid exception on MMU
> translation stage")
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/cpu_helper.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index bc70ab5abc..196166f8dd 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -1203,28 +1203,30 @@ static void raise_mmu_exception(CPURISCVState
> *env, target_ulong address,
>
>      switch (access_type) {
>      case MMU_INST_FETCH:
> -        if (env->virt_enabled && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_INST_ACCESS_FAULT;
> +        } else if (env->virt_enabled && !first_stage) {
>              cs->exception_index = RISCV_EXCP_INST_GUEST_PAGE_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_INST_ACCESS_FAULT : RISCV_EXCP_INST_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_INST_PAGE_FAULT;
>          }
>          break;
>      case MMU_DATA_LOAD:
> -        if (two_stage && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_LOAD_ACCESS_FAULT;
> +        } else if (two_stage && !first_stage) {
>              cs->exception_index = RISCV_EXCP_LOAD_GUEST_ACCESS_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_LOAD_ACCESS_FAULT : RISCV_EXCP_LOAD_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_LOAD_PAGE_FAULT;
>          }
>          break;
>      case MMU_DATA_STORE:
> -        if (two_stage && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_STORE_AMO_ACCESS_FAULT;
> +        } else if (two_stage && !first_stage) {
>              cs->exception_index = RISCV_EXCP_STORE_GUEST_AMO_ACCESS_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_STORE_AMO_ACCESS_FAULT :
> -                RISCV_EXCP_STORE_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_STORE_PAGE_FAULT;
>          }
>          break;
>      default:
> --
> 2.34.1
>
>

Digging more in  Priv-v1.12/riscv-privileged-20211203.pdf
<https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf>
 :
Page 82,  Section 4.3.2 Virtual Address Translation Process.

The spec actually mentions an address translation algorithm. Step 2
mentions the exception code should be " access-fault exception
corresponding to the original access type". i.e. 1, 5, 7.  All other steps
should use " page-fault exception corresponding to the original access
type". i.e. 12, 13, 15.

Regards,
Joseph Chan

On Mon, Apr 15, 2024 at 11:50 AM Joseph Chan <jchan@ventanamicro.com> wrote:

> FYI
>
> Priv-v1.12/riscv-privileged-20211203.pdf
> <https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf>
> defines exception priorities on
> Page 40, Table 3.7
> Page 130, Table 8.7
>
> There is a sentence under Table 3.7:
> "When a virtual address is translated into a physical address, the address
> translation algorithm
> determines what specific exception may be raised."
>
>
> The spec does not insist any implementation to report Exception Code 12
> over 1; 13,15 over 5, 7. On the other hand, the phrases "During instruction
> address translation:" and "With physical address for instruction:" gives me
> the impression that when the implementation can distinguish between these
> situations, then reporting 12 , 13, 15 instead of 1, 5, 7 will provide a
> fine-grained reason for why things were broken.
>
> Regards,
> Joseph Chan
>
>
> On Sat, Apr 13, 2024 at 3:59 AM Alexei Filippov <
> alexei.filippov@syntacore.com> wrote:
>
>> From: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
>>
>> raise_mmu_exception(), as is today, is prioritizing guest page faults by
>> checking first if virt_enabled && !first_stage, and then considering the
>> regular inst/load/store faults.
>>
>> There's no mention in the spec about guest page fault being a higher
>> priority that PMP faults. In fact, privileged spec section 3.7.1 says:
>>
>> "Attempting to fetch an instruction from a PMP region that does not have
>> execute permissions raises an instruction access-fault exception.
>> Attempting to execute a load or load-reserved instruction which accesses
>> a physical address within a PMP region without read permissions raises a
>> load access-fault exception. Attempting to execute a store,
>> store-conditional, or AMO instruction which accesses a physical address
>> within a PMP region without write permissions raises a store
>> access-fault exception."
>>
>> So, in fact, we're doing it wrong - PMP faults should always be thrown,
>> regardless of also being a first or second stage fault.
>>
>> The way riscv_cpu_tlb_fill() and get_physical_address() work is
>> adequate: a TRANSLATE_PMP_FAIL error is immediately reported and
>> reflected in the 'pmp_violation' flag. What we need is to change
>> raise_mmu_exception() to prioritize it.
>>
>> Reported-by: Joseph Chan <jchan@ventanamicro.com>
>> Fixes: 82d53adfbb ("target/riscv/cpu_helper.c: Invalid exception on MMU
>> translation stage")
>> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
>> ---
>>  target/riscv/cpu_helper.c | 22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
>> index bc70ab5abc..196166f8dd 100644
>> --- a/target/riscv/cpu_helper.c
>> +++ b/target/riscv/cpu_helper.c
>> @@ -1203,28 +1203,30 @@ static void raise_mmu_exception(CPURISCVState
>> *env, target_ulong address,
>>
>>      switch (access_type) {
>>      case MMU_INST_FETCH:
>> -        if (env->virt_enabled && !first_stage) {
>> +        if (pmp_violation) {
>> +            cs->exception_index = RISCV_EXCP_INST_ACCESS_FAULT;
>> +        } else if (env->virt_enabled && !first_stage) {
>>              cs->exception_index = RISCV_EXCP_INST_GUEST_PAGE_FAULT;
>>          } else {
>> -            cs->exception_index = pmp_violation ?
>> -                RISCV_EXCP_INST_ACCESS_FAULT :
>> RISCV_EXCP_INST_PAGE_FAULT;
>> +            cs->exception_index = RISCV_EXCP_INST_PAGE_FAULT;
>>          }
>>          break;
>>      case MMU_DATA_LOAD:
>> -        if (two_stage && !first_stage) {
>> +        if (pmp_violation) {
>> +            cs->exception_index = RISCV_EXCP_LOAD_ACCESS_FAULT;
>> +        } else if (two_stage && !first_stage) {
>>              cs->exception_index = RISCV_EXCP_LOAD_GUEST_ACCESS_FAULT;
>>          } else {
>> -            cs->exception_index = pmp_violation ?
>> -                RISCV_EXCP_LOAD_ACCESS_FAULT :
>> RISCV_EXCP_LOAD_PAGE_FAULT;
>> +            cs->exception_index = RISCV_EXCP_LOAD_PAGE_FAULT;
>>          }
>>          break;
>>      case MMU_DATA_STORE:
>> -        if (two_stage && !first_stage) {
>> +        if (pmp_violation) {
>> +            cs->exception_index = RISCV_EXCP_STORE_AMO_ACCESS_FAULT;
>> +        } else if (two_stage && !first_stage) {
>>              cs->exception_index =
>> RISCV_EXCP_STORE_GUEST_AMO_ACCESS_FAULT;
>>          } else {
>> -            cs->exception_index = pmp_violation ?
>> -                RISCV_EXCP_STORE_AMO_ACCESS_FAULT :
>> -                RISCV_EXCP_STORE_PAGE_FAULT;
>> +            cs->exception_index = RISCV_EXCP_STORE_PAGE_FAULT;
>>          }
>>          break;
>>      default:
>> --
>> 2.34.1
>>
>>

On Sat, Apr 13, 2024 at 9:00 PM Alexei Filippov
<alexei.filippov@syntacore.com> wrote:
>
> From: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
>
> raise_mmu_exception(), as is today, is prioritizing guest page faults by
> checking first if virt_enabled && !first_stage, and then considering the
> regular inst/load/store faults.
>
> There's no mention in the spec about guest page fault being a higher
> priority that PMP faults. In fact, privileged spec section 3.7.1 says:
>
> "Attempting to fetch an instruction from a PMP region that does not have
> execute permissions raises an instruction access-fault exception.
> Attempting to execute a load or load-reserved instruction which accesses
> a physical address within a PMP region without read permissions raises a
> load access-fault exception. Attempting to execute a store,
> store-conditional, or AMO instruction which accesses a physical address
> within a PMP region without write permissions raises a store
> access-fault exception."
>
> So, in fact, we're doing it wrong - PMP faults should always be thrown,
> regardless of also being a first or second stage fault.
>
> The way riscv_cpu_tlb_fill() and get_physical_address() work is
> adequate: a TRANSLATE_PMP_FAIL error is immediately reported and
> reflected in the 'pmp_violation' flag. What we need is to change
> raise_mmu_exception() to prioritize it.
>
> Reported-by: Joseph Chan <jchan@ventanamicro.com>
> Fixes: 82d53adfbb ("target/riscv/cpu_helper.c: Invalid exception on MMU translation stage")
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  target/riscv/cpu_helper.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index bc70ab5abc..196166f8dd 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -1203,28 +1203,30 @@ static void raise_mmu_exception(CPURISCVState *env, target_ulong address,
>
>      switch (access_type) {
>      case MMU_INST_FETCH:
> -        if (env->virt_enabled && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_INST_ACCESS_FAULT;
> +        } else if (env->virt_enabled && !first_stage) {
>              cs->exception_index = RISCV_EXCP_INST_GUEST_PAGE_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_INST_ACCESS_FAULT : RISCV_EXCP_INST_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_INST_PAGE_FAULT;
>          }
>          break;
>      case MMU_DATA_LOAD:
> -        if (two_stage && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_LOAD_ACCESS_FAULT;
> +        } else if (two_stage && !first_stage) {
>              cs->exception_index = RISCV_EXCP_LOAD_GUEST_ACCESS_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_LOAD_ACCESS_FAULT : RISCV_EXCP_LOAD_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_LOAD_PAGE_FAULT;
>          }
>          break;
>      case MMU_DATA_STORE:
> -        if (two_stage && !first_stage) {
> +        if (pmp_violation) {
> +            cs->exception_index = RISCV_EXCP_STORE_AMO_ACCESS_FAULT;
> +        } else if (two_stage && !first_stage) {
>              cs->exception_index = RISCV_EXCP_STORE_GUEST_AMO_ACCESS_FAULT;
>          } else {
> -            cs->exception_index = pmp_violation ?
> -                RISCV_EXCP_STORE_AMO_ACCESS_FAULT :
> -                RISCV_EXCP_STORE_PAGE_FAULT;
> +            cs->exception_index = RISCV_EXCP_STORE_PAGE_FAULT;
>          }
>          break;
>      default:
> --
> 2.34.1
>
>