On Fri, Feb 17, 2023, Oliver Upton wrote:
> Hi Yu,
>
> scripts/get_maintainers.pl is your friend for getting the right set of
> emails for a series :) Don't know about others, but generally I would
> prefer to be Cc'ed on an entire series (to gather context) than just an
> individual patch.
+1
>
> On Thu, Feb 16, 2023 at 09:12:28PM -0700, Yu Zhao wrote:
> > This patch adds kvm_arch_test_clear_young() for the vast majority of
> > VMs that are not pKVM and run on hardware that sets the accessed bit
> > in KVM page tables.
At least for the x86 changes, please read Documentation/process/maintainer-tip.rst
and rewrite the changelogs.
> > It relies on two techniques, RCU and cmpxchg, to safely test and clear
> > the accessed bit without taking the MMU lock. The former protects KVM
> > page tables from being freed while the latter clears the accessed bit
> > atomically against both the hardware and other software page table
> > walkers.
> >
> > Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 7 +++
> > arch/arm64/include/asm/kvm_pgtable.h | 8 +++
> > arch/arm64/include/asm/stage2_pgtable.h | 43 ++++++++++++++
> > arch/arm64/kvm/arm.c | 1 +
> > arch/arm64/kvm/hyp/pgtable.c | 51 ++--------------
> > arch/arm64/kvm/mmu.c | 77 ++++++++++++++++++++++++-
> > 6 files changed, 141 insertions(+), 46 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 35a159d131b5..572bcd321586 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -1031,4 +1031,11 @@ static inline void kvm_hyp_reserve(void) { }
> > void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu);
> > bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu);
> >
> > +/* see the comments on the generic kvm_arch_has_test_clear_young() */
Please eliminate all of these "see the comments on blah", in every case they do
nothing more than redirect the reader to something they're likely already aware of.
> > +#define kvm_arch_has_test_clear_young kvm_arch_has_test_clear_young
> > +static inline bool kvm_arch_has_test_clear_young(void)
> > +{
> > + return IS_ENABLED(CONFIG_KVM) && cpu_has_hw_af() && !is_protected_kvm_enabled();
> > +}
...
> Also, I'm at a loss for why we'd need to test if CONFIG_KVM is enabled.
> My expectation is that we should provide an implementation that returns
> false if !CONFIG_KVM, avoiding the need to repeat that bit in every
> single implementation of the function.
mm/vmscan.c uses kvm_arch_has_test_clear_young(). I have opinions on that, but
I'll hold off on expressing them until there's actual justification presented
somewhere.
Yu, this series and each patch needs a big pile of "why". I get that the goal
is to optimize memory oversubscribe, but there needs to be justification for
why this is KVM only, why nested VMs and !A/D hardware are out of scope, why yet
another mmu_notifier hook is being added, etc.
