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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
