On Wed, Feb 21, 2018 at 05:52:41PM +0000, Marc Zyngier wrote: > On 21/02/18 17:39, Andrew Jones wrote: > > On Thu, Feb 15, 2018 at 10:03:02PM +0100, Christoffer Dall wrote: > >> The debug save/restore functions can be improved by using the has_vhe() > >> static key instead of the instruction alternative. Using the static key > >> uses the same paradigm as we're going to use elsewhere, it makes the > >> code more readable, and it generates slightly better code (no > >> stack setups and function calls unless necessary). > >> > >> We also use a static key on the restore path, because it will be > >> marginally faster than loading a value from memory. > >> > >> Finally, we don't have to conditionally clear the debug dirty flag if > >> it's set, we can just clear it. > >> > >> Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx> > >> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx> > >> --- > >> > >> Notes: > >> Changes since v1: > >> - Change dot to comma in comment > >> - Rename __debug_restore_spe to __debug_restore_spe_nvhe > >> > >> arch/arm64/kvm/hyp/debug-sr.c | 26 ++++++++++++-------------- > >> 1 file changed, 12 insertions(+), 14 deletions(-) > >> > > > > Maybe after this series is merged, if there are any hyp_alternate_select's > > left, we can replace all the remaining ones with has_vhe() and then just > > completely remove hyp_alternate_select. > > Note that older compilers (such as GCC 4.8) will generate horrible code > with static keys, as they do not support "asm goto". Not that I want to > preserve the home brew hyp_alternate_select mechanism, but I just want > to make it plain that some distros will definitely suffer from the > transition. Yeah, I've seen that. I even wrote some patches to try and deal with it once, because RHEL currently has gcc 4.8, and static keys are now used in kernel hot paths for kpti, and I knew this series was coming. My patches didn't seem like something usptream would care about, so I never posted them. Indeed, I see here[*] that at least x86 is saying that at some point (soon?) asm-goto will be a hard requirement. I just checked Christoffer's branch. The only hyp_alternate_select, that couldn't be changed to a has_vhe is __check_arm_834220, but that one can just be changed to cpus_have_const_cap(ARM64_WORKAROUND_834220), since it's just acting as a boolean anyway. Thanks, drew [*] https://lkml.org/lkml/2018/2/20/51 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
