Re: [PATCH v3 0/5] x86/apic: Fix restoring boot irq mode in reboot and kexec/kdump

[Dou Liyang]

Hi Eric,

At 02/12/2018 01:11 PM, Eric W. Biederman wrote:
> Dou Liyang <douly.fnst@xxxxxxxxxxxxxx> writes:
>
> > Hi all,
> >
> > One thing confused me.
> >
> > The disconnect_bsp_APIC() may restore the interrupt delivery mode into
> > virtual wire mode. it uses the vector F as the spurious interrput, But,
> > IMO, using the vector 0xFF(SPURIOUS_APIC_VECTOR) may more suitable and
> > will give us more detail. Why the disconnect_bsp_APIC() use vector F
> > here?
>
> I would say this needs a documentation search before changing this.
>
> This code originates in:
> 208fb93162d5 ("[PATCH] kexec: x86_64: restore apic virtual wire mode on shutdown")
>
> The example in the Multi-Processor Specification v1.4 shows setting
> up the SPIV to vector 0x0f.
Thanks for your detailed explanation, I found the example A-1 in this
spec.

> I don't know what is canonical and what will interact best with DOS,
> and that erra of setup.  The vector 0x0f seems an odd choice as
> it is below 0x20 putting it in the range of vectors that are
> reserved for processor exceptions.
>
> The constant SPURIOUS_APIC_VECTOR is definitely not something we want
> to be using at this point as that is a linux specific setting and used
> when Linux is up and running.  So it is completely inapplicable.
>
> This is all about restoring how the apics were configured at boot time
> so it may be appropriate to copy and store this value, if it was not
> architectural.
>
> At a practical level at this point I suspect we are ok as the setting
> of the SPIV this way has not caused any known problems in the last
> decade.   If someone wants to dig through the archtectural documents
> and the real world practice and find a better value and explain the
> change I would not oppose it.

Indeed, I understand.


Thanks,
	dou

> All I know for certain is using the constant SPURIOUS_APIC_VECTOR
> is completely inappropriate (as that constant is about how linux uses
> vectors) and thus the patch below is wrong.
>
> > diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> > index 25ddf02598d2..550deaad6a9a 100644
> > --- a/arch/x86/kernel/apic/apic.c
> > +++ b/arch/x86/kernel/apic/apic.c
> > @@ -2130,7 +2130,7 @@ void disconnect_bsp_APIC(int virt_wire_setup)
> >          value = apic_read(APIC_SPIV);
> >          value &= ~APIC_VECTOR_MASK;
> >          value |= APIC_SPIV_APIC_ENABLED;
> > -       value |= 0xf;
> > +       value |= SPURIOUS_APIC_VECTOR;
> >          apic_write(APIC_SPIV, value);
> >
> >          if (!virt_wire_setup) {
>
> Eric