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SubjectRe: [PATCH v3 12/18] arm64: KVM: Add SMCCC_ARCH_WORKAROUND_1 fast handling
On Thu, Feb 01, 2018 at 11:46:51AM +0000, Marc Zyngier wrote:
> We want SMCCC_ARCH_WORKAROUND_1 to be fast. As fast as possible.
> So let's intercept it as early as we can by testing for the
> function call number as soon as we've identified a HVC call
> coming from the guest.

Hmmm. How often is this expected to happen and what is the expected
extra cost of doing the early-exit handling in the C code vs. here?

I think we'd be better off if we only had a single early-exit path (and
we should move the FP/SIMD trap to that path as well), but if there's a
measurable benefit of having this logic in assembly as opposed to in the
C code, then I'm ok with this as well.

The code in this patch looks fine otherwise.

Thanks,
-Christoffer

>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm64/kvm/hyp/hyp-entry.S | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index e4f37b9dd47c..f36464bd57c5 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -15,6 +15,7 @@
> * along with this program. If not, see <http://www.gnu.org/licenses/>.
> */
>
> +#include <linux/arm-smccc.h>
> #include <linux/linkage.h>
>
> #include <asm/alternative.h>
> @@ -64,10 +65,11 @@ alternative_endif
> lsr x0, x1, #ESR_ELx_EC_SHIFT
>
> cmp x0, #ESR_ELx_EC_HVC64
> + ccmp x0, #ESR_ELx_EC_HVC32, #4, ne
> b.ne el1_trap
>
> - mrs x1, vttbr_el2 // If vttbr is valid, the 64bit guest
> - cbnz x1, el1_trap // called HVC
> + mrs x1, vttbr_el2 // If vttbr is valid, the guest
> + cbnz x1, el1_hvc_guest // called HVC
>
> /* Here, we're pretty sure the host called HVC. */
> ldp x0, x1, [sp], #16
> @@ -100,6 +102,20 @@ alternative_endif
>
> eret
>
> +el1_hvc_guest:
> + /*
> + * Fastest possible path for ARM_SMCCC_ARCH_WORKAROUND_1.
> + * The workaround has already been applied on the host,
> + * so let's quickly get back to the guest. We don't bother
> + * restoring x1, as it can be clobbered anyway.
> + */
> + ldr x1, [sp] // Guest's x0
> + eor w1, w1, #ARM_SMCCC_ARCH_WORKAROUND_1
> + cbnz w1, el1_trap
> + mov x0, x1
> + add sp, sp, #16
> + eret
> +
> el1_trap:
> /*
> * x0: ESR_EC
> --
> 2.14.2
>

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