From: Nathan Lynch <nathanl@linux.ibm.com>
To: Alexey Kardashevskiy <aik@ozlabs.ru>, linuxppc-dev@lists.ozlabs.org
Cc: tyreld@linux.ibm.com, brking@linux.ibm.com, ajd@linux.ibm.com,
aneesh.kumar@linux.ibm.com
Subject: Re: [PATCH 5/6] powerpc/rtas: rename RTAS_RMOBUF_MAX to RTAS_USER_REGION_SIZE
Date: Tue, 19 Jan 2021 19:17:13 -0600 [thread overview]
Message-ID: <87y2gowgo6.fsf@linux.ibm.com> (raw)
In-Reply-To: <6905c3d2-e524-b6d8-036f-7812ea3f8b85@ozlabs.ru>
Alexey Kardashevskiy <aik@ozlabs.ru> writes:
> On 16/01/2021 02:56, Nathan Lynch wrote:
>> Alexey Kardashevskiy <aik@ozlabs.ru> writes:
>>> On 15/01/2021 09:00, Nathan Lynch wrote:
>>>> diff --git a/arch/powerpc/include/asm/rtas.h b/arch/powerpc/include/asm/rtas.h
>>>> index 332e1000ca0f..1aa7ab1cbc84 100644
>>>> --- a/arch/powerpc/include/asm/rtas.h
>>>> +++ b/arch/powerpc/include/asm/rtas.h
>>>> @@ -19,8 +19,11 @@
>>>> #define RTAS_UNKNOWN_SERVICE (-1)
>>>> #define RTAS_INSTANTIATE_MAX (1ULL<<30) /* Don't instantiate rtas at/above this value */
>>>>
>>>> -/* Buffer size for ppc_rtas system call. */
>>>> -#define RTAS_RMOBUF_MAX (64 * 1024)
>>>> +/* Work areas shared with RTAS must be 4K, naturally aligned. */
>>>
>>> Why exactly 4K and not (for example) PAGE_SIZE?
>>
>> 4K is a platform requirement and isn't related to Linux's configured
>> page size. See the PAPR specification for RTAS functions such as
>> ibm,configure-connector, ibm,update-nodes, ibm,update-properties.
>
> Good, since we are documenting things here - add to the comment ("per
> PAPR")?
But almost every constant in this header relates to a specification or
requirement in PAPR.
>> There are other calls with work area parameters where alignment isn't
>> specified (e.g. ibm,get-system-parameter) but 4KB alignment is a safe
>> choice for those.
>>
>>>> +#define RTAS_WORK_AREA_SIZE 4096
>>>> +
>>>> +/* Work areas allocated for user space access. */
>>>> +#define RTAS_USER_REGION_SIZE (RTAS_WORK_AREA_SIZE * 16)
>>>
>>> This is still 64K but no clarity why. There is 16 of something, what
>>> is it?
>>
>> There are 16 4KB work areas in the region. I can name it
>> RTAS_NR_USER_WORK_AREAS or similar.
>
>
> Why 16? PAPR (then add "per PAPR") or we just like 16 ("should be
> enough")?
PAPR doesn't know anything about the user region; it's a Linux
construct. It's been 64KB since pre-git days and I'm not sure what the
original reason is. At this point, maintaining a kernel-user ABI seems
like enough justification for the value.
next prev parent reply other threads:[~2021-01-20 1:19 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-14 21:59 [PATCH 0/6] powerpc/rtas: miscellaneous cleanups, user region allocation Nathan Lynch
2021-01-14 21:59 ` [PATCH 1/6] powerpc/rtas: improve ppc_rtas_rmo_buf_show documentation Nathan Lynch
2021-01-15 4:38 ` Alexey Kardashevskiy
2021-01-15 5:50 ` Andrew Donnellan
2021-01-14 22:00 ` [PATCH 2/6] powerpc/rtas-proc: remove unused RMO_READ_BUF_MAX Nathan Lynch
2021-01-15 4:38 ` Alexey Kardashevskiy
2021-01-15 5:52 ` Andrew Donnellan
2021-01-14 22:00 ` [PATCH 3/6] powerpc/rtas: remove ibm_suspend_me_token Nathan Lynch
2021-01-15 4:38 ` Alexey Kardashevskiy
2021-01-15 5:52 ` Andrew Donnellan
2021-01-14 22:00 ` [PATCH 4/6] powerpc/rtas: move syscall filter setup into separate function Nathan Lynch
2021-01-15 4:39 ` Alexey Kardashevskiy
2021-01-15 16:04 ` Nathan Lynch
2021-01-15 5:49 ` Andrew Donnellan
2021-01-14 22:00 ` [PATCH 5/6] powerpc/rtas: rename RTAS_RMOBUF_MAX to RTAS_USER_REGION_SIZE Nathan Lynch
2021-01-15 4:38 ` Alexey Kardashevskiy
2021-01-15 15:56 ` Nathan Lynch
2021-01-18 4:15 ` Alexey Kardashevskiy
2021-01-20 1:17 ` Nathan Lynch [this message]
2021-01-20 5:05 ` Alexey Kardashevskiy
2021-01-21 15:17 ` Nathan Lynch
2021-01-15 6:10 ` Andrew Donnellan
2021-01-15 12:04 ` kernel test robot
2021-01-14 22:00 ` [PATCH 6/6] powerpc/rtas: constrain user region allocation to RMA Nathan Lynch
2021-01-15 4:38 ` Alexey Kardashevskiy
2021-01-15 15:38 ` Nathan Lynch
2021-01-18 4:12 ` Alexey Kardashevskiy
2021-01-20 0:39 ` Nathan Lynch
2021-01-20 4:49 ` Alexey Kardashevskiy
2021-01-20 12:06 ` Michael Ellerman
2021-01-21 15:27 ` Nathan Lynch
2021-01-23 1:54 ` Alexey Kardashevskiy
2021-01-19 9:00 ` Michael Ellerman
2021-01-19 21:00 ` Nathan Lynch
2021-01-20 12:13 ` Michael Ellerman
2021-01-21 0:26 ` Nathan Lynch
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