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: Thu, 21 Jan 2021 09:17:51 -0600 [thread overview]
Message-ID: <87sg6uwc80.fsf@linux.ibm.com> (raw)
In-Reply-To: <7988dce5-6cf3-df79-1276-7bc91ce7c8b2@ozlabs.ru>
Alexey Kardashevskiy <aik@ozlabs.ru> writes:
> On 20/01/2021 12:17, Nathan Lynch wrote:
>> 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:
>>>>>> +#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.
>
> I am not arguing keeping the numbers but you are replacing one magic
> number with another and for neither it is horribly obvious where they
> came from.
When I wrote it I viewed it as changing one of the factors in (64 *
1024) to a named constant that better expresses how the region is used
and adjusting the remaining factor to arrive at the same end result. I
considered it a net improvement even if we're not sure how 64K was
arrived at in the first place, although I suspect it was chosen to
support multiple concurrent users, and to be compatible with both 4K
and 64K page sizes. Then again 64K pages came a bit after this was
introduced.
The change that introduced RTAS_RMOBUF_MAX (here renamed to
RTAS_USER_REGION_SIZE) does not explain how the value was derived:
================
Author: Andrew Morton <akpm@osdl.org>
Date: Sun Jan 18 18:17:30 2004 -0800
[PATCH] ppc64: add rtas syscall, from John Rose
From: Anton Blanchard <anton@samba.org>
Added RTAS syscall. Reserved lowmem rtas_rmo_buf for userspace use. Created
"rmo_buffer" proc file to export bounds of rtas_rmo_buf.
[...]
diff --git a/include/asm-ppc64/rtas.h b/include/asm-ppc64/rtas.h
index 42a0b484077c..d9e426161044 100644
--- a/include/asm-ppc64/rtas.h
+++ b/include/asm-ppc64/rtas.h
@@ -19,6 +19,9 @@
#define RTAS_UNKNOWN_SERVICE (-1)
#define RTAS_INSTANTIATE_MAX (1UL<<30) /* Don't instantiate rtas at/above this value */
+/* Buffer size for ppc_rtas system call. */
+#define RTAS_RMOBUF_MAX (64 * 1024)
+
================
The comment "Buffer size for ppc_rtas system call" (removed by my
change) is not really appropriate because 1. not all sys_rtas
invocations use the buffer, and 2. no callers use the entire buffer.
> Is 16 the max number of concurrently running sys_rtas system
> calls? Does the userspace ensure there is no more than 16?
No and no; not all calls to sys_rtas need to use a work area. However,
librtas uses record locking to arbitrate access to the user region, and
the unit of allocation is 4KB. This is a reasonable choice: many RTAS
calls which take a work area require 4KB alignment. But some do not
(ibm,get-system-parameter), and librtas conceivably could be made to
perform finer-grained allocations.
It's not the kernel's concern how librtas partitions the user region, so
I'm inclined to leave the (64 * 1024) expression alone now. Thanks for
your review.
> btw where is that userspace code? I thought
> https://github.com/power-ras/ppc64-diag.git but no. Thanks,
librtas, of which ppc64-diag and powerpc-utils are users:
https://github.com/ibm-power-utilities/librtas
next prev parent reply other threads:[~2021-01-21 15:20 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
2021-01-20 5:05 ` Alexey Kardashevskiy
2021-01-21 15:17 ` Nathan Lynch [this message]
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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