Re: [Qemu-devel] [PATCH] target/i386/kvm.c: Don't mark cpuid_data as QEMU_PACKED

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 12/10/18 12:46 PM, Peter Maydell wrote:
> clang complains about taking the address of a packed
> member of a struct:
> 
> target/i386/kvm.c:1245:27: warning: taking address of packed member 'cpuid' of class or structure '' may result in an unaligned pointer value [-Waddress-of-packed-member]
>     c = cpuid_find_entry(&cpuid_data.cpuid, 1, 0);
>                           ^~~~~~~~~~~~~~~~
> target/i386/kvm.c:1297:31: warning: taking address of packed member 'cpuid' of class or structure '' may result in an unaligned pointer value [-Waddress-of-packed-member]
>         c = cpuid_find_entry(&cpuid_data.cpuid, kvm_base, 0);
>                               ^~~~~~~~~~~~~~~~
> 
> The kernel's definitions of struct kvm_cpuid2 and struct
> kvm_cpuid_entry2 are carefully set up with padding fields
> so that there is no between-struct padding anyway, so
> the QEMU_PACKED annotation is unnecessary and might result
> in the compiler generating worse code. Drop it, and instead
> assert at build time that there is no stray padding.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> ---
>  target/i386/kvm.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index b2401d13ea7..739cf8c8ea1 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -864,7 +864,15 @@ int kvm_arch_init_vcpu(CPUState *cs)
>      struct {
>          struct kvm_cpuid2 cpuid;
>          struct kvm_cpuid_entry2 entries[KVM_MAX_CPUID_ENTRIES];
> -    } QEMU_PACKED cpuid_data;
> +    } cpuid_data;
> +    /*
> +     * The kernel defines these structs with padding fields so there
> +     * should be no extra padding in our cpuid_data struct.
> +     */
> +    QEMU_BUILD_BUG_ON(sizeof(cpuid_data) !=
> +                      sizeof(struct kvm_cpuid2) +
> +                      sizeof(struct kvm_cpuid_entry2) * KVM_MAX_CPUID_ENTRIES);
> +
>      X86CPU *cpu = X86_CPU(cs);
>      CPUX86State *env = &cpu->env;
>      uint32_t limit, i, j, cpuid_i;
>