Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 2 Feb 2019 09:28:42 -0800 | Subject | Re: [PATCH v5 07/13] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available |
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On Fri, Feb 1, 2019 at 12:54 PM Chang S. Bae <chang.seok.bae@intel.com> wrote: > > Copy real FS/GSBASE values instead of approximation when FSGSBASE is > enabled. > > Factoring out to save_fsgs() does not result in the same behavior because > save_base_legacy() does not copy FS/GSBASE when the index is zero. > > Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com> > Cc: Andy Lutomirski <luto@kernel.org> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Andi Kleen <ak@linux.intel.com> > --- > arch/x86/kernel/process_64.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c > index d8ade9530fdb..648e43b58c69 100644 > --- a/arch/x86/kernel/process_64.c > +++ b/arch/x86/kernel/process_64.c > @@ -477,10 +477,16 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp, > p->thread.sp = (unsigned long) fork_frame; > p->thread.io_bitmap_ptr = NULL; > > - savesegment(gs, p->thread.gsindex); > - p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase; > savesegment(fs, p->thread.fsindex); > - p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase; > + savesegment(gs, p->thread.gsindex); > + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { > + p->thread.fsbase = rdfsbase(); > + p->thread.gsbase = __rdgsbase_inactive(); > + } else { > + /* save_base_legacy() does not set base when index is zero. */
After looking at this a bit, I propose that we just clean this up all the way. Can't this whole mess be changed to:
save_fsgs(me); p->thread.fsindex = me->thread.fsindex; p->thread.fsbase = me->thread.fsbase; p->thread.gsindex = me->thread.gsindex; p->thread.gsbase = me->thread.gsbase;
This will avoid all of the horrible tracing through the logic to figure out why the code is correct.
Sure, it'll be a few cycles slower with FSGSBASE, but this isn't really a fast path, and if we ever really care, we can optimize it later.
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