Re: [PATCH] xen: x86: copy back tsc info, not pointer to tsc info in domctl

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/05/15 11:04, Ian Campbell wrote:
> In 38b37ed82705 "x86/domctl: cleanup", XEN_DOMCTL_gettscinfo was
> changed to use the standard copyback mechanism.
>
> However the output TSC Info is a guerst handle, i.e. a pointer to the
> location for the information, copyback just copies the unchanged
> pointer back.
>
> Switch back to fetching the details into a local struct and explicitly
> copying it back.
>
> This caused test failures in the Cambridge instance of osstest, but
> not for some reason in the production instance.
>
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>

Oops - I feel very silly for not noticing that.  My testing also didn't
notice this.

> ---
>  xen/arch/x86/domctl.c |   13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
> index 684259b..e7da735 100644
> --- a/xen/arch/x86/domctl.c
> +++ b/xen/arch/x86/domctl.c
> @@ -703,13 +703,16 @@ long arch_do_domctl(
>              ret = -EINVAL;
>          else
>          {
> +            xen_guest_tsc_info_t info = { 0 };
> +
>              domain_pause(d);
> -            tsc_get_info(d, &domctl->u.tsc_info.info.tsc_mode,
> -                         &domctl->u.tsc_info.info.elapsed_nsec,
> -                         &domctl->u.tsc_info.info.gtsc_khz,
> -                         &domctl->u.tsc_info.info.incarnation);
> +            tsc_get_info(d, &info.tsc_mode,
> +                            &info.elapsed_nsec,
> +                            &info.gtsc_khz,
> +                            &info.incarnation);
> +            if ( copy_to_guest(domctl->u.tsc_info.out_info, &info, 1) )
> +                ret = -EFAULT;
>              domain_unpause(d);

The copy_to_guest() can be after the domain_unpause().

Alternatively, being a domctl, we can change tsc_info to not be a guest
handle.  There is plenty of space inside the union, and it saves a
further memory access.

~Andrew