[PATCH v2 4/4] ARM: vfp: clear fpscr length and stride bits on entry to sig handler
Dave Martin
dave.martin at linaro.org
Mon Jan 30 12:07:03 EST 2012
On Mon, Jan 30, 2012 at 11:24:55AM +0000, Will Deacon wrote:
> The ARM PCS mandates that the length and stride bits of the fpscr are
> cleared on entry to and return from a public interface. Although signal
> handlers run asynchronously with respect to the interrupted function,
> the handler itself expects to run as though it has been called like a
> normal function.
>
> This patch updates the state mirroring the VFP hardware before entry to
> a signal handler so that it adheres to the PCS. Furthermore, we disable
> VFP to ensure that we trap on any floating point operation performed by
> the signal handler and synchronise the hardware appropriately. A check
> is inserted after the signal handler to avoid redundant flushing if VFP
> was not used.
Hmmm, that looks more like it. I've not tried this out, but it looks
sensible.
Ignore my stale comments on your previous post.
Cheers
---Dave
>
> Reported-by: Peter Maydell <peter.maydell at linaro.org>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> ---
> arch/arm/kernel/signal.c | 17 ++++++++++++++++-
> 1 files changed, 16 insertions(+), 1 deletions(-)
>
> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
> index 9e617bd..3de59a0 100644
> --- a/arch/arm/kernel/signal.c
> +++ b/arch/arm/kernel/signal.c
> @@ -207,6 +207,20 @@ static int preserve_vfp_context(struct vfp_sigframe __user *frame)
> __put_user_error(h->fpinst, &frame->ufp_exc.fpinst, err);
> __put_user_error(h->fpinst2, &frame->ufp_exc.fpinst2, err);
>
> + vfp_flush_hwstate(thread);
> +
> + /*
> + * As per the PCS, clear the length and stride bits before entry
> + * to the signal handler.
> + */
> + h->fpscr &= ~(FPSCR_LENGTH_MASK | FPSCR_STRIDE_MASK);
> +
> + /*
> + * Disable VFP so that we can detect if it was used by the
> + * signal handler.
> + */
> + h->fpexc &= ~FPEXC_EN;
> +
> return err ? -EFAULT : 0;
> }
>
> @@ -227,7 +241,8 @@ static int restore_vfp_context(struct vfp_sigframe __user *frame)
> if (magic != VFP_MAGIC || size != VFP_STORAGE_SIZE)
> return -EINVAL;
>
> - vfp_flush_hwstate(thread);
> + if (h->fpexc & FPEXC_EN)
> + vfp_flush_hwstate(thread);
>
> /*
> * Copy the floating point registers. There can be unused
> --
> 1.7.4.1
>
>
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