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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/05/15 11:18, Ian Campbell wrote:
> On Tue, 2015-05-26 at 11:13 +0100, Andrew Cooper wrote:
>> 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@xxxxxxxxxx>
>> Oops - I feel very silly for not noticing that.  My testing also didn't
>> notice this.
> It's pretty subtle.
>
> My best guess is that the h/w in Cambridge tends to be older and
> therefore perhaps doesn't all have synchronous TSCs or something.
>
>>> ---
>>>  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().
> So it can. 
>
>> 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.
> If you prefer, sure.
>
> Perhaps fix then cleanup in a separate patch?

Can do.

For this patch (subject to the reordering), Reviewed-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx>

>
> Ian.
>
>


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