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Subject: Re: [PATCH] x86, paravirt: BUG_ON on {rd,wr}msr exceptions
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 28 Jul 2014 13:43:36 -0700
To: Eduardo Habkost <ehabkost@redhat.com>
CC: virtualization@lists.linux-foundation.org,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        Chris Wright <chrisw@sous-sol.org>, Alok Kataria <akataria@vmware.com>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org
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As much as I hate adding new pvops, it might be the better answer, especially since those are the real native ops.


On July 28, 2014 1:39:55 PM PDT, Eduardo Habkost <ehabkost@redhat.com> wrote:
>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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