Re: [Xen-devel] [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Apr 4, 2016 4:51 AM, "Jan Kara" <jack@xxxxxxx> wrote:
>
> On Sat 02-04-16 13:58:19, Andy Lutomirski wrote:
> > [cc Jan Kara]
> >
> > On Sat, Apr 2, 2016 at 1:47 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> > > On Sat, Apr 02, 2016 at 01:13:37PM -0700, Andy Lutomirski wrote:
> > >> Given that I this isn't really a regression with my patches (it
> > >> probably never worked much better on 32-bit and the regs never would
> > >> have shown at all on 64-bit),
> > >
> > > You're right. That thing calls printk *and* early_printk, WTF:
> > >
> > > #ifdef CONFIG_EARLY_PRINTK
> > >
> > >         call early_printk
> > >         ...
> > >
> > >         call dump_stack
> > >
> > >         ...
> > >
> > >         call __print_symbol
> > >
> > > those last two call printk. Great.
> > >
> > >> I propose a different approach: make
> > >> printk work earlier.  Something like:
> > >>
> > >> if (early) {
> > >>     early_printk(args);
> > >> }
> > >>
> > >> or early_vprintk or whatever.
> > >>
> > >> If the cost of a branch mattered, this could be alternative-patched
> > >> out later on, but that seems silly.  I also bet that a more sensible
> > >> fallback could be created in which printk would try to use an early
> > >> console if there's no real console.
> > >
> > > So how about this:
> > >
> > > printk() does
> > >
> > >         vprintk_func = this_cpu_read(printk_func);
> > >
> > > and that's
> > >
> > > DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default
> > >
> > > I guess we can make that function be early_printk-something and once
> > > printk is initialized, we overwrite it with vprintk_default.
> > >
> > > Elegant and no need for if branches and alternatives.
> > >
> > > Hmmm.
> >
> > Jan, IIRC you were looking at printk recently-ish.  Any thoughts here?
>
> Sounds like a good idea to me. I've also consulted this with Petr Mladek
> (added to CC) who is using printk_func per-cpu variable in his
> printk-from-NMI patches and he also doesn't see a problem with this.
>
> I was just wondering about one thing - this way we add more early printks
> if I understand your intention right. Are we guaranteed that they happen
> only from a single CPU? Because currently there is no locking in
> early_printk() and thus we can end up writing to early console several
> messages in parallel from different CPUs. Not sure what's going to happen
> in that case...

Adding locking would be easy enough, wouldn't it?

But do any platforms really boot a second CPU before switching to real
printk?  Given that I see all the smpboot stuff in dmesg, I guess real
printk happens first.  I admit I haven't actually checked.

--Andy

>
>                                                                 Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

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