[2/3] firmware: arm_scmi: Remove unneeded __packed attribute
diff mbox series

Message ID 20200710133919.39792-2-cristian.marussi@arm.com
State Accepted
Commit 33ee97f823cc5b3d03c9910c1b8dbe193a21056b
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Series
  • [1/3] firmware: arm_scmi: Remove zero-length array in SCMI Notifications
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Commit Message

Cristian Marussi July 10, 2020, 1:39 p.m. UTC
Remove __packed attribute from struct scmi_event_header.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 drivers/firmware/arm_scmi/notify.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Steven Price July 13, 2020, 11:18 a.m. UTC | #1
On 10/07/2020 14:39, Cristian Marussi wrote:
> Remove __packed attribute from struct scmi_event_header.
> 
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>

A drive-by review. But this doesn't look safe to me. sizeof(struct 
scmi_event_header) is used in several places and this change will modify 
that from 13 to 16, but leave the structure members at the same offset 
(as the members are naturally aligned). In particular the naïve header 
size is now bigger than the offset to payld.

What is the justification for __packed being 'unneeded'? Perhaps there's 
some reason in the code why this doesn't matter, but if so I feel this 
should be included in the commit message.

Steve

> ---
>   drivers/firmware/arm_scmi/notify.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
> index c4d006cfde88..752415367305 100644
> --- a/drivers/firmware/arm_scmi/notify.c
> +++ b/drivers/firmware/arm_scmi/notify.c
> @@ -258,7 +258,7 @@ struct scmi_event_header {
>   	u8	evt_id;
>   	size_t	payld_sz;
>   	u8	payld[];
> -} __packed;
> +};
>   
>   struct scmi_registered_event;
>   
>
Steven Price July 13, 2020, 11:20 a.m. UTC | #2
On 10/07/2020 14:39, Cristian Marussi wrote:
> Remove __packed attribute from struct scmi_event_header.
> 
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>

A drive-by review. But this doesn't look safe to me. sizeof(struct 
scmi_event_header) is used in several places and this change will modify 
that from 13 to 16, but leave the structure members at the same offset 
(as the members are naturally aligned). In particular the naïve header 
size is now bigger than the offset to payld.

What is the justification for __packed being 'unneeded'?

Steve

> ---
>   drivers/firmware/arm_scmi/notify.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
> index c4d006cfde88..752415367305 100644
> --- a/drivers/firmware/arm_scmi/notify.c
> +++ b/drivers/firmware/arm_scmi/notify.c
> @@ -258,7 +258,7 @@ struct scmi_event_header {
>   	u8	evt_id;
>   	size_t	payld_sz;
>   	u8	payld[];
> -} __packed;
> +};
>   
>   struct scmi_registered_event;
>   
>
Cristian Marussi July 13, 2020, 1:07 p.m. UTC | #3
Hi Steven

thanks for the review.

On Mon, Jul 13, 2020 at 12:20:43PM +0100, Steven Price wrote:
> On 10/07/2020 14:39, Cristian Marussi wrote:
> >Remove __packed attribute from struct scmi_event_header.
> >
> >Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> 
> A drive-by review. But this doesn't look safe to me. sizeof(struct
> scmi_event_header) is used in several places and this change will modify
> that from 13 to 16, but leave the structure members at the same offset (as
> the members are naturally aligned). In particular the naïve header size is
> now bigger than the offset to payld.
> 
> What is the justification for __packed being 'unneeded'?
> 

Arnd pointed out at first that this structure in the original series had a mix of
fixed and non-fixed types in its fields and that the __packed rendered some fields
misaglined.

Removing that as it is, in fact left also some unexplained implicit padding which is
at odd for a struct containing fixed-sized types.

In a following fix in the series I have indeed moved this struct's fields  and others
to generic non fixed type fields and shuffled around the fields to avoid misalignment
and implicit internal padding (except for the trailing padding due to the variable
size array)

It was probably better to squash also this patch in that following patch.

This structure is used internally to push variable-sized (through the means of the payld[])
events descriptors through a fifo from the ISR to the deferred workqueus, so that's whhy I
originally thought to avoid to carry around unneeded padding into the fifos and use the
__packed.

On the correctness side, as you pointed out, the header with padding is now 16 so when
I push thorugh the kfifos this header and the payload there's a hole in the data as
represented in the fifo buffer as such

@end_of hdr+payld kfifo writes:
  kifo_in(fifo, h, sizeof(*h)) + kfifo_in(fifo, payld, h->payld_sz)

0       14   16
+-------+----+------------
|header - pad| payload...
-------------------------
        ^
	|
	.payld

(Note that header and payload comes from two distinct place so I have push it with two kfifo_in()
in order to avoid a redundant memcpy on an intermediate buffer to collate them...thing
that was pointed out as undesirable in a review)

and when I read it back from the fifo such hole is just transparently overwritten:

@header read:
 kfifo_out(fifo, h, sizeof(*h))

0       14   16
+-------+----+--------------
|header - pad| 
----------------------------

@payload_read:
 kfifo_out(fifo, h->payld, h->payld_sz)

0       14   
+-------+----+--------------
|header | payload....
----------------------------
        ^
	|
	.payld

So since anyway the drawback of packing is that the misaglined access potentially slows down the
reads, I was not sure anymore it was worth to pack and misalign, and, given that it seemed not
to be liked so much, I dropped it and moved to generic non-fixed types without packing.

A better (and shorter) explanation of all of the above is possibly needed (but I'd still prefer
the fixed sized typing and __packed 'holeless' approach...)

Thanks

Cristian


> Steve
> 
> >---
> >  drivers/firmware/arm_scmi/notify.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
> >index c4d006cfde88..752415367305 100644
> >--- a/drivers/firmware/arm_scmi/notify.c
> >+++ b/drivers/firmware/arm_scmi/notify.c
> >@@ -258,7 +258,7 @@ struct scmi_event_header {
> >  	u8	evt_id;
> >  	size_t	payld_sz;
> >  	u8	payld[];
> >-} __packed;
> >+};
> >  struct scmi_registered_event;
> >
>
Steven Price July 15, 2020, 10:56 a.m. UTC | #4
On 13/07/2020 14:07, Cristian Marussi wrote:
> Hi Steven
> 
> thanks for the review.
> 
> On Mon, Jul 13, 2020 at 12:20:43PM +0100, Steven Price wrote:
>> On 10/07/2020 14:39, Cristian Marussi wrote:
>>> Remove __packed attribute from struct scmi_event_header.
>>>
>>> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
>>
>> A drive-by review. But this doesn't look safe to me. sizeof(struct
>> scmi_event_header) is used in several places and this change will modify
>> that from 13 to 16, but leave the structure members at the same offset (as
>> the members are naturally aligned). In particular the na�ve header size is
>> now bigger than the offset to payld.
>>
>> What is the justification for __packed being 'unneeded'?
>>
> 
> Arnd pointed out at first that this structure in the original series had a mix of
> fixed and non-fixed types in its fields and that the __packed rendered some fields
> misaglined.
> 
> Removing that as it is, in fact left also some unexplained implicit padding which is
> at odd for a struct containing fixed-sized types.
> 
> In a following fix in the series I have indeed moved this struct's fields  and others
> to generic non fixed type fields and shuffled around the fields to avoid misalignment
> and implicit internal padding (except for the trailing padding due to the variable
> size array)
> 
> It was probably better to squash also this patch in that following patch.
> 
> This structure is used internally to push variable-sized (through the means of the payld[])
> events descriptors through a fifo from the ISR to the deferred workqueus, so that's whhy I
> originally thought to avoid to carry around unneeded padding into the fifos and use the
> __packed.
> 
> On the correctness side, as you pointed out, the header with padding is now 16 so when
> I push thorugh the kfifos this header and the payload there's a hole in the data as
> represented in the fifo buffer as such
> 
> @end_of hdr+payld kfifo writes:
>    kifo_in(fifo, h, sizeof(*h)) + kfifo_in(fifo, payld, h->payld_sz)
> 
> 0       14   16
> +-------+----+------------
> |header - pad| payload...
> -------------------------
>          ^
> 	|
> 	.payld
> 
> (Note that header and payload comes from two distinct place so I have push it with two kfifo_in()
> in order to avoid a redundant memcpy on an intermediate buffer to collate them...thing
> that was pointed out as undesirable in a review)
> 
> and when I read it back from the fifo such hole is just transparently overwritten:
> 
> @header read:
>   kfifo_out(fifo, h, sizeof(*h))
> 
> 0       14   16
> +-------+----+--------------
> |header - pad|
> ----------------------------
> 
> @payload_read:
>   kfifo_out(fifo, h->payld, h->payld_sz)
> 
> 0       14
> +-------+----+--------------
> |header | payload....
> ----------------------------
>          ^
> 	|
> 	.payld
> 
> So since anyway the drawback of packing is that the misaglined access potentially slows down the
> reads, I was not sure anymore it was worth to pack and misalign, and, given that it seemed not
> to be liked so much, I dropped it and moved to generic non-fixed types without packing.

Thanks for the explanation - it sounds like the change is correct.

However, from the description above it sounds like splitting the header 
and payload into separate types would be clearer. I'm not sure the 
flexible length array is adding to code clarity here. In particular the 
'pad' being put into the fifo is actually going to be a (truncated) copy 
of the payload.

There is also a trick with an unnamed internal struct which gets the 
padding in the correct place...

	struct scmi_event_header {
		struct {
			u64	timestamp;
			u8	evt_id;
			size_t	payld_sz;
		}
		u8	payld[];
	};

...with that then...

	offsetof(struct scmi_event_header, payld) ==
		sizeof(struct scmi_event_header)

...which avoids the need for kfifo_out having to overwrite the padding.

> A better (and shorter) explanation of all of the above is possibly needed (but I'd still prefer
> the fixed sized typing and __packed 'holeless' approach...)

Fixed sized types and __packed is easier to reason about, but obviously 
naturally aligned types do tend to be faster.

Steve

Patch
diff mbox series

diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
index c4d006cfde88..752415367305 100644
--- a/drivers/firmware/arm_scmi/notify.c
+++ b/drivers/firmware/arm_scmi/notify.c
@@ -258,7 +258,7 @@  struct scmi_event_header {
 	u8	evt_id;
 	size_t	payld_sz;
 	u8	payld[];
-} __packed;
+};
 
 struct scmi_registered_event;