Re: [Xen-devel] [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Mon, Mar 14, 2016 at 11:04 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> None of this insane complicated crap that buys us exactly *nothing*,
> and depends on fancy new exception handling support etc etc.

Actually, the one _new_ thing we could do is to instead of removing
the old crappy rdmsr()/wrmsr() implementation entirely, we'll just
rename it to "rd/wrmsr_unsafe()", and not have any exception table
stuff for that at all (so now you *will* get an oops and panic if you
use that).

The only reason to do that is for performance-critical MSR's that
really don't want any overhead. Sure, they could just be changed to
use "wrmsr_safe()", but for things like the APIC accesses, or
update_debugctlmsr() (that is supposed to check for processor version)
that can be truly critical, an explicitly _unsafe_ version may be the
right thing to do.

The fact is, the problem with rd/wrmsr is that we just did the
defaults the wrong way around. Making the simple and obvious version
be unsafe is just wrong.

              Linus

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