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: <CA+55aFy4Chd-PJvxGne=TztPsvFqUG07Ht8mtc4+0oGNZNDOyw@mail.gmail.com>
Date:	Sun, 20 Sep 2015 17:15:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KVM list <kvm@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops

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.

                   Linus
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