Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

["H. Peter Anvin" <hpa@zytor.com>]

On 10/16/2013 05:00 AM, Jan Beulich wrote:
> Having had reports of certain Windows versions, when put in some
> special driver verification mode, blue-screening due to the FPU state
> having changed across interrupt handler runs (resulting from a host/
> hypervisor side context switch somewhere in the middle of the guest
> interrupt handler execution) on Xen, and assuming that KVM would suffer
> from the same problem, as well as having also noticed (long ago) that
> 32-bit processes don't behave correctly in this regard when run on a
> 64-bit kernel, this is the resulting attempt to port (and suitably
> extend) the Xen side fix to Linux.
> 
> The basic idea here is to either use a priori information on the
> intended state layout (in the case of 32-bit processes) or "sense" the
> proper layout (in the case of KVM guests) by inspecting the already
> saved FPU rip/rdp, and reading their actual values in a second save
> operation.
> 
> This second save operation could be another [F]XSAVE, but on all
> systems I measured this on using FNSTENV turned out to be the faster
> alternative.

It is not at all clear to me from the description what the flow is that
causes the problem, whatever the problem is.  Perhaps it should be if I
wasn't horribly sleep-deprived, but the description should be clear
enough that one should be able to tell the problem at a glance.

Please describe the flow that causes trouble.

Is this basically a problem with the 32-bit version of FXSAVE versus the
64-bit version?

Furthermore, you define X86_FEATURE_NO_FPU_SEL, but you don't set it
anywhere.  At least that bit needs to be factored out into a separate patch.

+	if (config_enabled(CONFIG_IA32_EMULATION) &&
+	    test_tsk_thread_flag(tsk, TIF_IA32))

is_ia32_task()?

	-hpa