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From: "H. Peter Anvin" <hpa@xxxxxxxxxxxxxxx>

When we write entropy into a non-empty pool, we currently don't

account at all for the fact that we will probabilistically overwrite

some of the entropy in that pool. This means that unless the pool is

fully empty, we are currently *guaranteed* to overestimate the amount

of entropy in the pool!

This version of the patchset avoids manually duplicating information

by using a macro. This removes *all* dynamic computation of derived

pool information and replaces them with static information: on just

about every architecture accessing pointer+offset is no more expensive

than just plain pointer, and this lets us get the information we

actually need from the start.

This version of the patchset adds handling of fractional bits, so that

we can continue to add a single bit of entropy without it being

rounded down to zero. This version has 3 bits of fraction, which

means that with a 4096-bit input pool the multiply still cannot

overflow 32 bits; if we want to add the capability of crediting

fractional bits of entropy, which may be useful in itself, then we

probably need more bits of fraction and would have to use a 64-bit

multiply and shift.

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