On Wed, 2016-10-05 at 09:14 +0200, Paolo Bonzini wrote: > > On 05/10/2016 02:34, [email protected] wrote: > > > > From: Andy Lutomirski <[email protected]> > > > > Since commit 58122bf1d856 ("x86/fpu: Default eagerfpu=on on all > > CPUs") in Linux 4.6, eager FPU mode has been the default on all x86 > > systems, and no one has reported any regressions. > > > > This patch removes the ability to enable lazy mode: use_eager_fpu() > > becomes "return true" and all of the FPU mode selection machinery > > is > > removed. > > I haven't quite followed up on my promise to benchmark lazy vs. eager > FPU, but I probably should do that now... > > I see two possible issues with this. First, AMD as far as I know > does > not have XSAVEOPT. Second, when using virtualization, depending on > how > you configure your cluster it's enough to have one pre-SandyBridge > Intel > machine to force no XSAVE on all machines.
The "OPT" part of XSAVEOPT does not work across the host/guest boundary, anyway. One of the items used in the tuple that determines whether the optimization can be used is whether or not the system is in the VMX root, or in a guest. In other words, across a VMEXIT / VMENTER boundary, it does full saves & restores, if I am reading the manual right. -- All Rights Reversed.
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