Hi Paolo,
Thank you to review the patch!
On Aug 18, 2014, at 9:57 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> Il 14/08/2014 09:01, Xiao Guangrong ha scritto:
>> - update_memslots(slots, new, kvm->memslots->generation);
>> + /* ensure generation number is always increased. */
>> + slots->generation = old_memslots->generation;
>> + update_memslots(slots, new);
>> rcu_assign_pointer(kvm->memslots, slots);
>> synchronize_srcu_expedited(&kvm->srcu);
>> + slots->generation++;
>
> I don't trust my brain enough to review this patch.
Sorry to make you confused. I should expain it more clearly.
What this patch tried to fix is: kvm will generate wrong mmio-exit forever
if no luck event cleans mmio spte. (eg. if no memory pressure or no
context-sync on that spte.)
Note, it is hard to do precise sync between kvm_vm_ioctl_set_memory_region
and mmio-exit - that means userspace is able to get mmio-exit even if
kvm_vm_ioctl_set_memory_region have finished, for example, kvm identifies
a mmio access before issuing the ioctl and injects mmio-exit to userspace after
ioctl return. So checking if mmio-exit is a real mmio access in userspace is
needed anyway.
> kvm_current_mmio_generation seems like a very bad (race-prone) API. One
> patch I trust myself reviewing would change a bunch of functions in
> kvm_main.c to take a memslots struct. This would make it easy to
> respect the hard and fast rule of not dereferencing the same pointer
> twice. But it would be a tedious change.
kvm_set_memory_region is the only place updating memslot and
kvm_current_mmio_generation accesses memslot by rcu-dereference,
i do not know why other places need to take into account.
I think this patch is auditable, page-fault is always called by holding
srcu-lock so that a page fault can’t go across synchronize_srcu_expedited.
Only these cases can happen:
1) page fault occurs before synchronize_srcu_expedited.
In this case, vcpu will generate mmio-exit for the memslot being registered
by the ioctl. That’s ok since the ioctl have not finished.
2) page fault occurs after synchronize_srcu_expedited and during
increasing generation-number.
In this case, userspace may get wrong mmio-exit (that happen if handing
page-fault is slower that the ioctl), that’s ok too since userspace need do
the check anyway like i said above.
3) page fault occurs after generation-number update
that’s definitely correct. :)
> Another alternative could be to use the low bit to mark an in-progress
> change, and skip the caching if the low bit is set. Similar to a
> seqcount (except if read_seqcount_retry fails, we just punt and not
> retry anything), you could use it even though the memory barriers
> provided by write_seqcount_begin/end are not too useful in this case.
I do not know how the bit works, page fault will cache the memslot before
the bit set and cache the generation-number after the bit set.
Maybe i missed your idea, could you please detail it?
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