Re: [Qemu-devel] [PATCH] target-i386: GPF on invalid MSRs

[Alexander Graf]

On 27.05.2011, at 17:13, Josh Triplett wrote:

> On Thu, May 26, 2011 at 11:12:12AM +0200, Alexander Graf wrote:
>> On 26.05.2011, at 11:08, Josh Triplett wrote:
>>> qemu currently returns 0 for rdmsr on invalid MSRs, and ignores wrmsr on
>>> invalid MSRs.  Real x86 processors GPF on invalid MSRs, which allows
>>> software to detect unavailable MSRs.  Emulate this behavior correctly in
>>> qemu.
>>> 
>>> Bug discovered via the BIOS Implementation Test Suite
>>> <http://biosbits.org/>; fix tested the same way, for both 32-bit and
>>> 64-bit x86.
>> 
>> This would break a _lot_ of guests that work just fine today, as qemu 
>> doesn't handle all the necessary MSRs.
> 
> It also fixes guests that rely on the GPF to indicate the absence of an
> MSR, and assume that the lack of GPF means the availability of that MSR.
> Silently returning 0 for unknown MSRs means silent breakage.

It's not about guests triggereing MSRs that they shouldn't. It's that qemu 
doesn't implement all MSRs that all the respective CPUs implement.

> 
> What (buggy) guests expect to use random model-specific registers
> without either handling GPFs or checking the CPU model first?

Mac OS X for example :). It even breaks on KVM today due to MSR checks.

> 
> What MSRs do those guests expect that qemu doesn't currently implement?
> 
> If this represents a workaround for buggy guests, then may I add an
> option to control this behavior?

I'm not against this change per-se, but it should definitely have an option to 
disable/enable it and you need to do very extensive testing to make sure that 
all MSRs for most OSs are actually handled.


Alex