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.zyng...@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 >