Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

From: H. Peter Anvin
Date: Wed May 21 2008 - 20:33:02 EST

Suresh Siddha wrote:

An ugly workaround could be to start clearing one of these fields,
and say that the data there is only valid for kernels >= 2.6.26.
(I said it was ugly...)

Or we go back to stashing a flag in uc_flags (which is kosher),
and try to figure out how to mark non-rt sigframes.

This issue of not-zeroing, is present in only 64bit kernels and for 64bit apps,

64bit app signal handling uses only rt_frame, so we can add an uc_flag for
them and for 32bit apps, kernel was always zero'ing the reserved bits
at the end of _fpstate.

In short, for non-rt frames, they can check the reserved bits at the end
of fpstate frame and for rt-frames (perhaps even for 32bit rt frame handling)
apps can check for uc_flag aswell, for extended state presence. Is this
good enough?

Okay, trying to close on this :)

I would suggest using the uc_flag for the rt frames, and simply rely on the OSXSAVE flag for non-rt signal frames. It a rather sucky approach (as previously discussed), but since any sane user of these fields (as opposed to just relying on the kernel to save/restore) should use the SIGINFO frames, I don't see a problem *as long as it's possible to get the information* -- any solution which demands performance should just turn on SIGINFO and be happy.

The biggest potential problem with this that I see is that relying on CPUID can mess with certain virtualization solutions. Another option to accomplish the same thing would be to have a system call (preferrably a prctl, since it is at least in theory personality-dependent) to query what information is included in the fpstate data - since it will always be the same for any particular kernel.



To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at