Andi Kleen wrote:
Even then it's useless for many flags because any user program can (and will)
call CPUID directly.
Turns out not to matter, since userspace can only make use of
capabilities that are already available to userspace. If the feature
bits for system features are visible to it, it doesn't really matter.
Yes, this could be broken in some cases. But it turns out to be safe.
Even sysenter support, which userspace does care about, is done via
setting the vsyscall page up in the kernel, rather than userspace CPUID
detection.
Sure the point was if they write this long fancy document why stop
at documenting the last 5%?
Because the last 5% is what is changing to meet Xen's needs. Why
document something that you know you are going to break in a week? I
chose to document the stable interfaces first.
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