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