Re: [PATCH 0/2] Enable -Wvla, forbidding use of variable length arrays

[Peter Maydell]

On Wed, 31 Jan 2024 at 14:56, Thomas Huth <thuth@redhat.com> wrote:
> There's still a vla left in the ppc kvm code:
>
>   https://gitlab.com/thuth/qemu/-/jobs/6063230079#L2005
>
> ../target/ppc/kvm.c: In function ‘kvmppc_save_htab’:
> ../target/ppc/kvm.c:2691:5: error: ISO C90 forbids variable length array
> ‘buf’ [-Werror=vla]
>   2691 |     uint8_t buf[bufsize];
>        |     ^~~~~~~
> ../target/ppc/kvm.c: In function ‘kvmppc_read_hptes’:
> ../target/ppc/kvm.c:2773:9: error: ISO C90 forbids variable length array
> ‘buf’ [-Werror=vla]
>   2773 |         char buf[sizeof(*hdr) + m * HASH_PTE_SIZE_64];
>        |         ^~~~
> cc1: all warnings being treated as errors

Thanks for catching that -- it being in code built only on
ppc hosts I missed it.

kvm_ppc_save_htab() is called twice, and in both cases the
bufsize passed in is MAX_KVM_BUF_SIZE. So we could drop
that argument and have the buf[] array always be MAX_KVM_BUF_SIZE.

kvmppc_read_hptes() does this:
        int m = n < HPTES_PER_GROUP ? n : HPTES_PER_GROUP;
        char buf[sizeof(*hdr) + m * HASH_PTE_SIZE_64];

HPTES_PER_GROUP is 8 and HASH_PTE_SIZE_64 is 16, so we aren't
saving many bytes of stack by trying to make the buf smaller
based on the value of n. So we could have the buf always
be [sizeof(*hdr) + HPTES_PER_GROUP * HASH_PTE_SIZE_64].

thanks
-- PMM