Re: [PATCH] x86, paravirt: BUG_ON on {rd,wr}msr exceptions

From: Eduardo Habkost
Date: Mon Jul 28 2014 - 16:40:23 EST

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().

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