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

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Message-ID: <CALCETrXAAOcGP7DK+7aKn=2pu=SQ0n_PhG9bV4DcoYcv9epn4A@mail.gmail.com>
Date:	Mon, 14 Mar 2016 11:24:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops

On Mon, Mar 14, 2016 at 11:15 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Mar 14, 2016 at 11:10 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> A couple of the wrmsr users actually care about performance.  These
>> are the ones involved in context switching and, to a lesser extent, in
>> switching in and out of guest mode.
>
> .. ok, see the crossed emails.
>
> I'd *much* rather special case the special cases. Not make the generic
> case something complex.

The code in my queue is, literally:

bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
                 struct pt_regs *regs, int trapnr)
{
    WARN_ONCE(1, "unchecked MSR access error: RDMSR from 0x%x",
          (unsigned int)regs->cx);

    /* Pretend that the read succeeded and returned 0. */
    regs->ip = ex_fixup_addr(fixup);
    regs->ax = 0;
    regs->dx = 0;
    return true;
}
EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);

The only regard in which this is any more complex than the _safe
variant is because there's a warning (one line of code) and because
the _safe variant forgot to zero the result (two lines of code).  My
series fixes the latter, so we're talking about almost no source code
size difference.  There *is* a difference in binary size, though --
the _safe variant emits a copy of its fixup every time it appears,
whereas the new fixup appears once.

So I think we should apply my patches (with the early handling fixed
and the panic_on_oops removed), and then consider reimplementing the
_safe variant using fancy handlers to reduce number of lines of asm
and code size, and then consider changing the overall API on top if we
think there's a better API to be had.

Is that okay?

>
> And *particularly* not make the generic case be something where people
> think it's sane to oops and kill the machine. Christ.

I've already removed the panic_on_oops thing from my tree.

--Andy