linux-kernel - Re: [PATCH 2/2] drivers/perf: riscv: Do not allow invalid raw event config

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Message-ID: <c094ae0b-ddf0-42fc-ad47-00fad84fb70d@sifive.com>
Date: Wed, 11 Dec 2024 22:06:11 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Atish Kumar Patra <atishp@...osinc.com>,
 Palmer Dabbelt <palmer@...belt.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, atishp@...shpatra.org,
 anup@...infault.org, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>, mchitale@...tanamicro.com,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drivers/perf: riscv: Do not allow invalid raw event
 config

Hi Atish,

On 2024-12-11 3:10 PM, Atish Kumar Patra wrote:
> On Wed, Dec 11, 2024 at 11:46 AM Palmer Dabbelt <palmer@...belt.com> wrote:
>>
>> On Mon, 09 Dec 2024 16:04:46 PST (-0800), Atish Patra wrote:
>>> The SBI specification allows only lower 48bits of hpmeventX to be
>>> configured via SBI PMU. Currently, the driver masks of the higher
>>> bits but doesn't return an error. This will lead to an additional
>>> SBI call for config matching which should return for an invalid
>>> event error in most of the cases.
>>>
>>> However, if a platform(i.e Rocket and sifive cores) implements a
>>> bitmap of all bits in the event encoding this will lead to an
>>> incorrect event being programmed leading to user confusion.
>>>
>>> Report the error to the user if higher bits are set during the
>>> event mapping itself to avoid the confusion and save an additional
>>> SBI call.
>>>
>>> Suggested-by: Samuel Holland <samuel.holland@...ive.com>
>>> Signed-off-by: Atish Patra <atishp@...osinc.com>
>>> ---
>>>  drivers/perf/riscv_pmu_sbi.c | 10 ++++++----
>>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
>>> index 3473ba02abf3..fb6eda90f771 100644
>>> --- a/drivers/perf/riscv_pmu_sbi.c
>>> +++ b/drivers/perf/riscv_pmu_sbi.c
>>> @@ -507,7 +507,7 @@ static int pmu_sbi_event_map(struct perf_event *event, u64 *econfig)
>>>  {
>>>       u32 type = event->attr.type;
>>>       u64 config = event->attr.config;
>>> -     int ret;
>>> +     int ret = -ENOENT;
>>>
>>>       /*
>>>        * Ensure we are finished checking standard hardware events for
>>> @@ -536,8 +536,11 @@ static int pmu_sbi_event_map(struct perf_event *event, u64 *econfig)
>>>
>>>               switch (config >> 62) {
>>>               case 0:
>>> -                     ret = RISCV_PMU_RAW_EVENT_IDX;
>>> -                     *econfig = config & RISCV_PMU_RAW_EVENT_MASK;
>>> +                     /* Return error any bits [48-63] is set  as it is not allowed by the spec */
>>> +                     if (!(config & ~RISCV_PMU_RAW_EVENT_MASK)) {
>>> +                             *econfig = config & RISCV_PMU_RAW_EVENT_MASK;
>>> +                             ret = RISCV_PMU_RAW_EVENT_IDX;
>>> +                     }
>>>                       break;
>>>               case 2:
>>>                       ret = (config & 0xFFFF) | (SBI_PMU_EVENT_TYPE_FW << 16);
>>> @@ -554,7 +557,6 @@ static int pmu_sbi_event_map(struct perf_event *event, u64 *econfig)
>>>               }
>>>               break;
>>>       default:
>>> -             ret = -ENOENT;
>>>               break;
>>>       }
>>
>> This doesn't have a Fixes, is it
>>
>>     Fixes: f0c9363db2dd ("perf/riscv-sbi: Add platform specific firmware event handling")
>>
> 
> I was not sure if a Fixes tag was worth it as the current
> behavior(masking off the higher bits) is there from the beginning of
> the driver.
> perf tool throws a warning as well if a user tries to set any of the
> upper 16 bits as event attributes is set to 0-47.
> 
> If it should be backported, this is the correct fixes tag.
> Fixes: e9991434596f ("RISC-V: Add perf platform driver based on SBI
> PMU extension")

I would agree the masking isn't a real issue before changing the event attribute
from 48 to 56 bits wide. However, this patch also does a drive-by fix of a
different bug introduced by f0c9363db2dd: there is no "case 1" or default case,
so the function would return an uninitialized value if the high bits are 01.
Maybe that should be a separate patch, with a Fixes tag?

Regards,
Samuel