Re: [Xen-devel] [PATCH] [v3] x86: Convert x86_platform_ops to timespec64

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

On 04/28/2018 11:09 AM, Arnd Bergmann wrote:
> On Sat, Apr 28, 2018 at 12:21 AM, Joao Martins
> <joao.m.martins@xxxxxxxxxx> wrote:
>> On 04/27/2018 09:13 PM, Arnd Bergmann wrote:
>>> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
>>> index 761f6af6efa5..637982efecd8 100644
>>> --- a/arch/x86/kernel/pvclock.c
>>> +++ b/arch/x86/kernel/pvclock.c
>>> @@ -123,28 +123,35 @@ u64 pvclock_clocksource_read(struct 
>>> pvclock_vcpu_time_info *src)
>>>
>>>  void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
>>>                           struct pvclock_vcpu_time_info *vcpu_time,
>>> -                         struct timespec *ts)
>>> +                         struct timespec64 *ts)
>>>  {
>>>       u32 version;
>>>       u64 delta;
>>> -     struct timespec now;
>>> +     struct timespec64 now;
>>>
>>>       /* get wallclock at system boot */
>>>       do {
>>>               version = wall_clock->version;
>>>               rmb();          /* fetch version before time */
>>> +             /*
>>> +              * Note: wall_clock->sec is a u32 value, so it can
>>> +              * only store dates between 1970 and 2106. To allow
>>> +              * times beyond that, we need to create a new hypercall
>>> +              * interface with an extended pvclock_wall_clock structure
>>> +              * like ARM has.
>>> +              */
>>>               now.tv_sec  = wall_clock->sec;
>>
>> IIUC the interface you're probably speaking about is common to both ARM and 
>> x86
>> on Xen[*] (since Xen 4.6) i.e.
>>
>>         now.tv_sec  = ((uint64_t)s->wc_sec_hi << 32) | s->wc_sec;
>>
>> s representing struct shared_info like on ARM (there's a 32-bit hole where
>> wc_sec_hi is placed on x86_64/ARM). Except on x86 32-bit guests wc_sec_hi is
>> located elsewhere.
>>
>>         Joao
>>
>> [*]
>> https://xenbits.xen.org/docs/4.6-testing/hypercall/x86_64/include,public,xen.h.html#incontents_startofday_shared
> 
> Ah, good. How portable is that? Will it do the right thing (i.e.
> guarantee to have
> zeroes on the upper half, or the epoch if supported) on all versions of both 
> KVM
> and Xen, or do we need an additional check in there?
>
The whole shared info page is zeroed out by Xen when allocated, so on
unsupported platforms that includes the upper half. But I don't know if this is
considered ABI or not. FWIW, the oldest release (2.0) has that behavior.

But this is Xen that I'm speaking about; KVM doesn't support this IIUC.

On KVM, there's HC_CLOCK_PAIRING hypercall or else *maybe* host could just write
wc_sec_hi at the end of the wall_clock struct (with the current MSR) and given
that it's (PAGE_SIZE aligned) guest memory, guest could always keep it zeroed
out for unsupported platforms (that won't write more than 12bytes).

> I'd suggest leaving the implementation of that to a follow-up patch that you
> can add once my patch is merged.

/nods

        Joao

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