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SubjectRe: [PATCH 5/6] x86/mm/kaiser: Optimize RESTORE_CR3
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.

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