Re: [PATCH] KVM: Fix a warning in __kvm_gfn_to_hva_cache_init()

[Sean Christopherson]

On Mon, May 04, 2020 at 03:05:26PM -0400, Peter Xu wrote:
> GCC 10.0.1 gives me this warning when building KVM:
> 
>   warning: ânr_pages_availâ may be used uninitialized in this function [-Wmaybe-uninitialized]
>   2442 |  for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
> 
> It should not happen, but silent it.

Heh, third times a charm?  This has been reported and proposed twice
before[1][2].  Are you using any custom compiler flags?  E.g. -O3 is known
to cause false positives with -Wmaybe-uninitialized.

If we do end up killing this warning, I'd still prefer to use
uninitialized_var() over zero-initializing the variable.

[1] https://lkml.kernel.org/r/20200218184756.242904-1-oupton@xxxxxxxxxx
[2] https://bugzilla.kernel.org/show_bug.cgi?id=207173

> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 74bdb7bf3295..2da293885a67 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2425,7 +2425,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
>  	gfn_t start_gfn = gpa >> PAGE_SHIFT;
>  	gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
>  	gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
> -	gfn_t nr_pages_avail;
> +	gfn_t nr_pages_avail = 0;
>  
>  	/* Update ghc->generation before performing any error checks. */
>  	ghc->generation = slots->generation;
> -- 
> 2.26.2
>