linux-kernel - Re: [PATCH] x86, paravirt: BUG

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Message-ID: <20140728203955.GA4611@thinpad.lan.raisama.net>
Date:	Mon, 28 Jul 2014 17:39:55 -0300
From:	Eduardo Habkost <ehabkost@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	virtualization@...ts.linux-foundation.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Alok Kataria <akataria@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, paravirt: BUG_ON on {rd,wr}msr exceptions

On Mon, Jul 28, 2014 at 12:18:10PM -0700, H. Peter Anvin wrote:
> On 07/28/2014 12:04 PM, Eduardo Habkost wrote:
> > When CONFIG_PARAVIRT is enabled, the kernel is ignoring exceptions on
> > the {rd,wr}msr instructions. This makes serious issues (either on the
> > guest kernel, or on the host) be silently ignored, and is different from
> > the native MSR code (which does not ignore the exceptions).
> > 
> > As paravirt.h already includes linux/bug.h, I don't see what was the
> > original issue preventing BUG_ON from being used.
> > 
> > Change rdmsr(), wrmsr(), and rdmsrl() to BUG_ON() on errors.
> 
> How much does this bloat the kernel?

It seems to add 8 bytes to each {wr,rd}msr() call (4 extra instructions:
test, jmp, ud2, jmp).

allyesconfig, paravirt enabled, before:

  text    data     bss     dec     hex filename
108368312       23500872        55705600        187574784       b2e2a00 vmlinux

allyesconfig, paravirt enabled, after:

   text    data     bss     dec     hex filename
108384438       23500904        55717888        187603230       b2e991e vmlinux

allyesconfig vmlinux is 28446 bytes larger.

An alternative is to add read_msr_unsafe() & write_msr_unsafe() fields
to pv_cpu_ops, pointing to native_read_msr() & native_write_msr().

-- 
Eduardo
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