Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1495888

Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode

Show all headers | View raw

On 05/10/2016 15:57, Rik van Riel wrote:
> On Wed, 2016-10-05 at 09:14 +0200, Paolo Bonzini wrote:
>>
>> On 05/10/2016 02:34, riel@redhat.com wrote:
>>>
>>> From: Andy Lutomirski <luto@kernel.org>
>>>
>>> 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.

Yes, but it works for bare metal (and in fact eager FPU was keyed on
XSAVEOPT before 58122bf1d856, not XSAVE).

I'm not talking about KVM here; I am just saying that the lazy FPU code
might be used more than we'd like to, because of AMD machines and of
cases where XSAVE is hidden altogether from guests.  Of course it is
quite unlikely that it be reported as a regression, since things just
work.  But as far as I know 58122bf1d856 went in without any substantial
(or not-so-substantial) benchmarking.

Paolo

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode riel@redhat.com - 2016-10-05 02:40 +0200
  Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode Paolo Bonzini <pbonzini@redhat.com> - 2016-10-05 09:20 +0200
    Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode Rik van Riel <riel@redhat.com> - 2016-10-05 16:00 +0200
      Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode Paolo Bonzini <pbonzini@redhat.com> - 2016-10-05 16:10 +0200
        Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode Andy Lutomirski <luto@amacapital.net> - 2016-10-05 18:10 +0200
          Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode Paolo Bonzini <pbonzini@redhat.com> - 2016-10-05 18:20 +0200
  [tip:x86/fpu] x86/fpu: Hard-disable lazy FPU mode tip-bot for Andy Lutomirski <tipbot@zytor.com> - 2016-10-07 11:50 +0200

csiph-web