[PATCH] kexec: Add --lite option

[panand@redhat.com (Pratyush Anand)]

Hi James,

Thanks for the reply.

On 07/12/2015:01:16:06 PM, James Morse wrote:
> Hi Pratyush,
> 
> On 07/12/15 11:48, Pratyush Anand wrote:
> >> 1) When we execute kexec() system call in first kernel, at that time it
> >> calculates sha256 on all the binaries [1]. It take almost un-noticeable time
> >> (less than a sec) there.
> >>
> >> 2) When purgatory is executed then it re-calculates sha256 using same routines
> >> [2] on same binary data as that of case (1). But, now it takes 10-20 sec
> >> (depending of size of binaries)?
> >>
> >> Why did not it take same time with O2 + D-cache enabled? I think, we should be
> >> able to achieve same time in second case as well. What is missing?
> 
> I haven't benchmarked this, but:
> 
> util_lib/sha256.c contains calls out to memcpy().
> In your case 1, this will use the glibc version. In case 2, it will use
> the version implemented in purgatory/string.c, which is a byte-by-byte copy.
> 

Yes, I agree that byte copy is too slow. But, memcpy() in sha256_update() will
copy only few bytes (I think max 126 bytes). Most of the data will be processed
using loop while( length >= 64 ){}, where we do not have any memcpy.So, I do not
think that this would be causing such a difference.

Could it be the case that I am not using perfect memory attributes while setting
up identity mapping and enabling D-cache. My implementation is here:
https://github.com/pratyushanand/kexec-tools/commit/8efdbc56b52f99a8a074edd0ddc519d7b68be82f

~Pratyush