Messages in this thread | | | From | Paolo Bonzini <> | Subject | Re: [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with SRCU | Date | Fri, 7 May 2021 10:42:19 +0200 |
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On 06/05/21 20:42, Ben Gardon wrote: > In preparation for lazily allocating the rmaps when the TDP MMU is in > use, protect the rmaps with SRCU. Unfortunately, this requires > propagating a pointer to struct kvm around to several functions.
Thinking more about it, this is not needed because all reads of the rmap array are guarded by the load-acquire of kvm->arch.memslots_have_rmaps. That is, the pattern is always
if (!load-acquire(memslot_have_rmaps)) return; ... = __gfn_to_rmap(...)
slots->arch.rmap[x] = ... store-release(memslot_have_rmaps, true)
where the load-acquire/store-release have the same role that srcu_dereference/rcu_assign_pointer had before this patch.
We also know that any read that misses the check has the potential for a NULL pointer dereference, so it *has* to be like that.
That said, srcu_dereference has zero cost unless debugging options are enabled, and it *is* true that the rmap can disappear if kvm->srcu is not held, so I lean towards keeping this change and just changing the commit message like this:
--------- Currently, rmaps are always allocated and published together with a new memslot, so the srcu_dereference for the memslots array already ensures that the memory pointed to by slots->arch.rmap is zero at the time slots->arch.rmap. However, they still need to be accessed in an SRCU read-side critical section, as the whole memslot can be deleted outside SRCU. --------
Thanks,
Paolo
> > Suggested-by: Paolo Bonzini<pbonzini@redhat.com> > Signed-off-by: Ben Gardon<bgardon@google.com> > --- > arch/x86/kvm/mmu/mmu.c | 57 +++++++++++++++++++++++++----------------- > arch/x86/kvm/x86.c | 6 ++--- > 2 files changed, 37 insertions(+), 26 deletions(-)
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