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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

Sorry, the title is not clear enough.

This is the v2 which fixes the issue pointed out by David:
" the generation number actually decreases."

Please review.

On 08/14/2014 03:01 PM, Xiao Guangrong wrote:
> We may cache the current mmio generation number and stale memslot info
> into spte, like this scenario：
> 
>        CPU 0                                  CPU 1
> page fault:                                add a new memslot
> read memslot and detecting its a mmio access
>                                            update memslots
>                                            update generation number
> read generation number
> cache the gpa and current gen number into spte
> 
> So, if guest accesses the gpa later, it will generate a incorrect
> mmio exit
> 
> This patch fixes it by updating the generation number after
> synchronize_srcu_expedited() that makes sure the generation
> number updated only if memslots update is finished
> 
> Cc: stable@vger.kernel.org
> Cc: David Matlack <dmatlack@google.com>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
>  virt/kvm/kvm_main.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 33712fb..bb40df3 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -96,7 +96,7 @@ static void hardware_disable_all(void);
> 
>  static void kvm_io_bus_destroy(struct kvm_io_bus *bus);
>  static void update_memslots(struct kvm_memslots *slots,
> -			    struct kvm_memory_slot *new, u64 last_generation);
> +			    struct kvm_memory_slot *new);
> 
>  static void kvm_release_pfn_dirty(pfn_t pfn);
>  static void mark_page_dirty_in_slot(struct kvm *kvm,
> @@ -687,8 +687,7 @@ static void sort_memslots(struct kvm_memslots *slots)
>  }
> 
>  static void update_memslots(struct kvm_memslots *slots,
> -			    struct kvm_memory_slot *new,
> -			    u64 last_generation)
> +			    struct kvm_memory_slot *new)
>  {
>  	if (new) {
>  		int id = new->id;
> @@ -699,8 +698,6 @@ static void update_memslots(struct kvm_memslots *slots,
>  		if (new->npages != npages)
>  			sort_memslots(slots);
>  	}
> -
> -	slots->generation = last_generation + 1;
>  }
> 
>  static int check_memory_region_flags(struct kvm_userspace_memory_region *mem)
> @@ -722,9 +719,12 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
>  {
>  	struct kvm_memslots *old_memslots = kvm->memslots;
> 
> -	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++;
> 
>  	kvm_arch_memslots_updated(kvm);
>