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

[Josh Triplett]

On Fri, May 27, 2011 at 05:16:56PM +0200, Alexander Graf wrote:
> 
> 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.

Ah, of course, since they only run on their own hardware.  Fair enough.

> > 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.

Fair enough.  Expect PATCHv2 with an option in the near future.

- Josh Triplett