Re: [Xen-devel] [PATCH v8 1/5] x86: add simple udelay calibration

[Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>]

Hi, Lu

At 07/13/2017 09:17 AM, Lu Baolu wrote:
> Hi,
>
> On 07/12/2017 04:02 PM, Dou Liyang wrote:
> > Hi, Lu
> >
> > At 05/05/2017 08:50 PM, Boris Ostrovsky wrote:
> > > On 05/05/2017 01:41 AM, Lu Baolu wrote:
> > > > Hi,
> > > >
> > > > On 05/03/2017 06:38 AM, Boris Ostrovsky wrote:
> > > > > On 03/21/2017 04:01 AM, Lu Baolu wrote:
> > > > > > Add a simple udelay calibration in x86 architecture-specific
> > > > > > boot-time initializations. This will get a workable estimate
> > > > > > for loops_per_jiffy. Hence, udelay() could be used after this
> > > > > > initialization.
> > > > > This breaks Xen PV guests since at this point, and until
> > > > > x86_init.paging.pagetable_init() which is when pvclock_vcpu_time_info is
> > > > > mapped, they cannot access pvclock.
> > > > >
> > > > > Is it reasonable to do this before tsc_init() is called? (The failure
> > > > > has nothing to do with tsc_init(), really --- it's just that it is
> > > > > called late enough that Xen PV guests get properly initialized.) If it
> > > > > is, would it be possible to move simple_udelay_calibration() after
> > > > > x86_init.paging.pagetable_init()?
> > > > This is currently only used for bare metal. How about by-pass it
> > > > for Xen PV guests?
> > > It is fixed this for Xen PV guests now (in the sense that we don't crash
> > > anymore) but my question is still whether this is not too early. Besides
> > > tsc_init() (which might not be important here), at the time when
> > > simple_udelay_calibration() is invoked we haven't yet called:
> > > * kvmclock_init(), which sets calibration routines for KVM
> > > * init_hypervisor_platform(), which sets calibration routines for vmware
> > > and Xen HVM
> > > * x86_init.paging.pagetable_init(), which sets calibration routines for
> > > Xen PV
> > I guess these may have been missed.
> >
> > Do you have any comments about these?
> The patch will be available in 4.13-rc1.
Yes, I have seen it in the upstream.

Firstly, I also met this problem want to call udelay() earlier than
*loops_per_jiffy* setup like you[1]. So I am very interesting in this
patch. ;)

I am also confused about the questions which Boris asked:

whether do the CPU and TSC calibration too early just for using
udelay()?

this design broke our interface of x86_paltform.calibrate_cpu/tsc.

And I also have a question below.

[...]

> > > > > > +static void __init simple_udelay_calibration(void)
> > > > > > +{
> > > > > > +    unsigned int tsc_khz, cpu_khz;
> > > > > > +    unsigned long lpj;
> > > > > > +
> > > > > > +    if (!boot_cpu_has(X86_FEATURE_TSC))
> > > > > > +        return;
if we don't have the TSC feature in booting CPU and
it returns here,  can we use udelay() correctly like before?


[1] https://lkml.org/lkml/2017/7/3/276

Thanks,

        dou.

> > Thanks,
> >
> >     dou.
> >
> > > > > > +
> > > > > > +    cpu_khz = x86_platform.calibrate_cpu();
> > > > > > +    tsc_khz = x86_platform.calibrate_tsc();
> > > > > > +
> > > > > > +    tsc_khz = tsc_khz ? : cpu_khz;
> > > > > > +    if (!tsc_khz)
> > > > > > +        return;
> > > > > > +
> > > > > > +    lpj = tsc_khz * 1000;
> > > > > > +    do_div(lpj, HZ);
> > > > > > +    loops_per_jiffy = lpj;
> > > > > > +}
> > > > > > +
> > > > > >  /*
> > > > > >   * Determine if we were loaded by an EFI loader.  If so, then we have also been
> > > > > >   * passed the efi memmap, systab, etc., so we should use these data structures
> > > > > > @@ -985,6 +1005,8 @@ void __init setup_arch(char **cmdline_p)
> > > > > >       */
> > > > > >      x86_configure_nx();
> > > > > >
> > > > > > +    simple_udelay_calibration();
> > > > > > +
> > > > > >      parse_early_param();
> > > > > >
> > > > > >  #ifdef CONFIG_MEMORY_HOTPLUG
> >
> > --
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