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

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 [off-list ref] wrote:

On Mon, Jul 28, 2014 at 12:18:10PM -0700, H. Peter Anvin wrote:

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On 07/28/2014 12:04 PM, Eduardo Habkost wrote:

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When CONFIG_PARAVIRT is enabled, the kernel is ignoring exceptions

on

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the {rd,wr}msr instructions. This makes serious issues (either on

the

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guest kernel, or on the host) be silently ignored, and is different

from

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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

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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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