On Wed, Nov 29, 2017 at 09:02:12PM +0100, Borislav Petkov wrote: > On Wed, Nov 29, 2017 at 11:33:06AM +0100, Peter Zijlstra wrote: > > +.macro RESTORE_CR3 scratch_reg:req save_reg:req > > STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1 > > + > > + /* ASID bit 11 is for user */ > > + bt $11, \save_reg > > <---- newline here.
Seems weird to me, the bt and jnc are a pair. > > + /* > > + * KERNEL pages can always resume with NOFLUSH as we do > > + * explicit flushes. > > + */ > > + jnc .Lnoflush_\@ > > + > > + /* > > + * Check if there's a pending flush for the user ASID we're > > + * about to set. > > + */ > > + movq \save_reg, \scratch_reg > > + andq $(0x7FF), \scratch_reg > > + bt \scratch_reg, PER_CPU_VAR(user_asid_flush_mask) > > + jnc .Lnoflush_\@ > > + > > + btr \scratch_reg, PER_CPU_VAR(user_asid_flush_mask) > > + jmp .Ldo_\@ > > Can you save yourself one of the BT-insns? > > ... > andq $(0x7FF), \scratch_reg > btr \scratch_reg, PER_CPU_VAR(user_asid_flush_mask) > jnc .Lnoflush_\@ > jmp .Ldo_\@ > ... > > or am I missing a case? BTR is an unconditional write and will modify the line and cause a write-back later. The common case is the bit not set, so BT, which is a pure read, avoids all that overhead.
