linux-kernel - Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio generation number

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Date:	Mon, 18 Aug 2014 15:57:39 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>, gleb@...nel.org
CC:	avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio
 generation number

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.

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.

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.

Paolo
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