At 8:21 AM -0500 10/27/99, Gabriel Paubert wrote:
[...]

And why would you want to use a bitmap ? This seems braindead to me, put a
value between 0 and 32 in vrsave. Since all registers are identical
in use and purpose, save registers 0 to n. Disclaimer: I've not seen if
the ABI specifies how and which Altivec registers are saved restored
across calls.

It would seem to me that using a bitmap to mark used registers would allow
more flexibility on the compiler side to play with register usage without
incurring longer context switch times. I wouldn't try to guess the relative
tradeoff values though.

Paranoid point of view: the restore must reload all altivec registers
(or clear the ones which are not specified as used by VRSAVE), otherwise
you might leak the contents of the Altivec registers of another process.
I'm not a security expert, but I don't like this possibility at all.

[...]

Beyond security, clearning the registers would also serve to enforce strict
usage of whatever is defined as the VRSAVE format, and avoid the
possibility of a mouth-breathing code-typist releasing a binary that
doesn't mark it's registers, which in theory would only break once a
different application touches the altivec registers, resulting in a
situation of either A: the kernel being forced to save all altivec
registers (bad) or B: allowing those binaries to be broken and upsetting
it's users (slightly less bad). Obviously pre-breaking those binaries would
be the preferable solution, so they never need see the light of day.


That's my 2c.

Cheers - Tony :)


--
Tony Mantler         Renaissance Nerd Extraordinaire         eek@escape.ca
Winnipeg, Manitoba, Canada                       http://www.escape.ca/~eek


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