The stack size is something that applies to addresses when performing
accesses as part of some instructions. This was handled inconsistently
or incompletely or simply incorrectly in a few ways.
First, when pushing or popping from the stack, the *address size* should
be set to the stack size. The data size is generally the operand size.
When the stack pointer is incremented/decremented, it should be changed
by the data size. When a stack pointer is manipulated, the data size
for those calculations should be the stack size. Importantly that does
not change the value of the increment/decrement, which is the operand
size still. This usage has been fixed throughout.
The TLB generally needs to know what the address size was in order to
figure out what segment offset was used so that it can do limit checks.
There is some inherent inaccuracy in doing things in reverse like this,
but that's how it works currently. To find that size, the TLB tried to
start from first principles to figure out what the default address size
was, and then whether there was an override was passed in through the
request flags.
This is *very* inaccurate for a few reasons. First, the override doesn't
always apply. Second, the address size used by a particular instruction
doesn't have to be based on any particular size, whether that is the
default or alternate address size, the stack size, etc.
Instead, the instructions now pass the actual size being used in as a 2
bit value (0 -> 1 byte, 1 -> 2 bytes, 2 -> 4 bytes, 3 -> 8 bytes),
avoiding most of the inaccuracy and approximation.
Because the CPU won't embed any size information into fetches, we'll
just assume those have no wrap around within the address size.
Finally, there were microops that had been added which overrode the
address size to be the stack size internally, and try to help the TLB
figure out what to do to figure out the address size. Because both of
those things are now handled in a different way, those microops are no
longer needed or used and have been deleted.
Change-Id: I2b1bdf1acf1540bf643fac6d49fe1a5a576ba5c1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55443
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
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