On Oct. 18, 2007, 10:06 +0200, Matthew Wilcox <matthew@xxxxxx> wrote:
> On Thu, Oct 18, 2007 at 08:59:58AM +0200, Benny Halevy wrote:
>> yeah. The sglist pointer shuffle makes sense and so are the field type
>> changes and coalescing, but the union holding the deprecated fields
>> of scsi_data_buff is going away.
>
> Indeed. We could always do ...
>
> union {
> struct scsi_data_pointer;
> struct {
> char _padding_[sizeof(struct scsi_data_pointer)];
> int result;
> }
> }
>
> But that might set a new record of depths of obfuscation.
agreed :)
>
>> #pragma pack(4) before struct scsi_cmnd
>> (followed by #pragma pack()) should do the trick
>> by saving member padding bytes. As a general rule I don't like pragma's
>> but this one works, is easy to understand, and is pretty standard.
>
> But it might well have the effect of pessimising accesses to the struct,
> and accesses to scsi_cmnd are pretty hot. I've seen gcc revert to byte
> loads and stores to packed structs in the past. Maybe it's got better
> now.
>
that's why I suggested pack(4) and not pack(1), assuming 4-byte alignment
is still as efficient as 8-byte alignment on most common 64 bit architectures.
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