Re: [Xen-devel] [BUG] x86-64 floating point environment handling

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 Nov 2005, at 15:35, Jan Beulich wrote:

> Besides the minor problem of the save/restore code needlessly being
> conditional for x86-64 (cpu_has_fxsr should always be set for all 
> 64-bit
> CPUs) there is a more significant problem: Both FXSAVE and FXRSTOR
> require, for a 64-bit environment, 64-bit operand size to be explicitly
> used. Simply adding a rex64/ prefix, however, doesn't work because
The only difference between rex64-prefixed fxsave and non-prefixed 
fxsave is the format of the last-instruction/data pointers. Does anyone 
make use of those? I notice that Linux always uses the rex64-prefixed 
version of fxsave, so I guess not (at least, not in 32-bit apps running 
on 64-bit linux).
It's very unlikely we'd implement expensive and complicated logic in 
Xen to ensure consistency of FP state that noone uses.
 -- Keir


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