Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear_young()

[Sean Christopherson <seanjc@google.com>]

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@google.com>
> > ---
> >  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.