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SubjectRe: [PATCH v4 1/2] arm64: adjust el0_sync so that a function can be called
On Thu, May 22, 2014 at 11:35:20PM +0100, Larry Bassel wrote:
> > On 05/22/2014 03:27 PM, Larry Bassel wrote:
> > > To implement the context tracker properly on arm64,
> > > a function call needs to be made after debugging and
> > > interrupts are turned on, but before the lr is changed
> > > to point to ret_to_user(). If the function call
> > > is made after the lr is changed the function will not
> > > return to the correct place.
> > >
> > > For similar reasons, defer the setting of x0 so that
> > > it doesn't need to be saved around the function call
> > > (save far_el1 in x26 temporarily instead).
> > >
> > > Signed-off-by: Larry Bassel <larry.bassel@linaro.org>
> > > ---
> > > arch/arm64/kernel/entry.S | 24 +++++++++++++++++-------
> > > 1 file changed, 17 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > > index e8b23a3..20b336e 100644
> > > --- a/arch/arm64/kernel/entry.S
> > > +++ b/arch/arm64/kernel/entry.S
> > > @@ -354,7 +354,6 @@ el0_sync:
> > > lsr x24, x25, #ESR_EL1_EC_SHIFT // exception class
> > > cmp x24, #ESR_EL1_EC_SVC64 // SVC in 64-bit state
> > > b.eq el0_svc
> > > - adr lr, ret_to_user
> > > cmp x24, #ESR_EL1_EC_DABT_EL0 // data abort in EL0
> > > b.eq el0_da
> > > cmp x24, #ESR_EL1_EC_IABT_EL0 // instruction abort in EL0
> > > @@ -383,7 +382,6 @@ el0_sync_compat:
> > > lsr x24, x25, #ESR_EL1_EC_SHIFT // exception class
> > > cmp x24, #ESR_EL1_EC_SVC32 // SVC in 32-bit state
> > > b.eq el0_svc_compat
> > > - adr lr, ret_to_user
> > > cmp x24, #ESR_EL1_EC_DABT_EL0 // data abort in EL0
> > > b.eq el0_da
> > > cmp x24, #ESR_EL1_EC_IABT_EL0 // instruction abort in EL0
> > > @@ -426,22 +424,26 @@ el0_da:
> > > /*
> > > * Data abort handling
> > > */
> > > - mrs x0, far_el1
> > > - bic x0, x0, #(0xff << 56)
> > > + mrs x26, far_el1
> > > // enable interrupts before calling the main handler
> > > enable_dbg_and_irq
> > > + mov x0, x26
> > > + bic x0, x0, #(0xff << 56)
> >
> > Nit: I believe you can bit clear with x26 as the source register and omit the
> > move instruction.
>
> Is that really an improvement (assuming it works)? Are we saving
> any cycles here? If so, does it matter? It is easy to see what
> the move instruction is doing.

Even if it's not noticeable, I would still reduce the number of lines by
one. BIC with immediate is just an alias for AND and it supports
different source and destination.

--
Catalin


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