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Re: [Qemu-devel] [PATCH 4/6] crypto: convert xts_tweak_encdec to use xts


From: Alberto Garcia
Subject: Re: [Qemu-devel] [PATCH 4/6] crypto: convert xts_tweak_encdec to use xts_uint128 type
Date: Tue, 09 Oct 2018 17:30:25 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

>> >      for (i = 0; i < lim; i++) {
>> > -        xts_tweak_encdec(datactx, decfunc, src, dst, (uint8_t *)&T);
>> > +        xts_uint128 S, D;
>> > +
>> > +        memcpy(&S, src, XTS_BLOCK_SIZE);
>> > +        xts_tweak_encdec(datactx, decfunc, &S, &D, &T);
>> > +        memcpy(dst, &D, XTS_BLOCK_SIZE);
>> 
>> Why do you need S and D?
>
> I think src & dst pointers can't be guaranteed to be aligned
> sufficiently for int64 operations, if we just cast from uint8t*.

I see. I did a quick test without the memcpy() calls and it doesn't seem
to have a visible effect on performance, but if it turns out that it
does then maybe this is worth investigating further. I suspect all
buffers received by this code are allocated with qemu_try_blockalign()
anyway, so it should be safe.

Berto



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