linux-kernel - Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on

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Message-ID: <CALCETrXqsKcqzfqMO2ctOJJ4X8Kb6vvoUpVDir9F2-a3XwK23w@mail.gmail.com>
Date:	Sun, 20 Sep 2015 18:13:15 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops

On Sep 20, 2015 5:15 PM, "Linus Torvalds" <torvalds@...ux-foundation.org> wrote:
>
> On Sun, Sep 20, 2015 at 5:02 PM, Andy Lutomirski <luto@...nel.org> wrote:
> > This demotes an OOPS and likely panic due to a failed non-"safe" MSR
> > access to a WARN_ON_ONCE and a return of zero (in the RDMSR case).
> > We still write a pr_info entry unconditionally for debugging.
>
> No, this is wrong.
>
> If you really want to do something like this, then just make all MSR
> reads safe. So the only difference between "safe" and "unsafe" is that
> the unsafe version just doesn't check the return value, and silently
> just returns zero for reads (or writes nothing).
>
> To quote Obi-Wan: "Use the exception table, Luke".
>
> Because decoding instructions is just too ugly. We'll do it for CPU
> errata where we might have to do it for user space code too (ie the
> AMD prefetch mess), but for code that _we_ control? Hell no.
>
> So NAK on this.

My personal preference is to just not do this at all.  A couple people
disagree.  If we make the unsafe variants not oops, then I think we
want to have the nice loud warning, since these issues are bugs if
they happen.

We could certainly use the exception table for this, but it'll result
in bigger core, since each MSR access will need an exception table
entry and an associated fixup to call some helper that warns and sets
the result to zero.

I'd be happy to implement that, but only if it'll be applied.
Otherwise I'd rather just drop this patch and keep the rest of the
series.

--Andy
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