Re: [PATCH -v2] Add MCE support to KVM

From: Gregory Haskins <gregory.haskins@gmail.com>
To: Kyle Moffett <kyle@moffetthome.net>
Cc: Huang Ying <ying.huang@intel.com>,
	Anthony Liguori <anthony@codemonkey.ws>,
	Avi Kivity <avi@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH -v2] Add MCE support to KVM
Date: Tue, 21 Apr 2009 22:42:11 -0400	[thread overview]
Message-ID: <49EE8403.3090204@gmail.com> (raw)
In-Reply-To: <f73f7ab80904192057q3d918ceco81c09febb493e1c3@mail.gmail.com>

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Kyle Moffett wrote:
> On Sun, Apr 19, 2009 at 9:19 PM, Huang Ying <ying.huang@intel.com> wrote:
>   
>> On Sat, 2009-04-18 at 23:54 +0800, Anthony Liguori wrote:
>>     
>>> Huang Ying wrote:
>>>       
>>>> The related MSRs are emulated. MCE capability is exported via
>>>> extension KVM_CAP_MCE and ioctl KVM_X86_GET_MCE_CAP_SUPPORTED. A new
>>>> vcpu ioctl command KVM_X86_SETUP_MCE is used to setup MCE emulation
>>>> such as the mcg_cap. MCE is injected via vcpu ioctl command
>>>> KVM_X86_SET_MCE. Extended machine-check state (MCG_EXT_P) and CMCI are
>>>> not simulated.
>>>>
>>>>         
>>> Maybe I'm missing something, but couldn't this be implemented entirely
>>> within userspace?  There's nothing VT/SVM specific about this.  If the
>>> issue is setting these MSRs from userspace via KVM_SET_MSRS isn't
>>> enough, perhaps we should add userspace MSR handling.
>>>
>>> Also, if you implement the MSR logic in userspace, it's pretty simple to
>>> make it work in the non-TCG case which will be a requirement for
>>> upstream merging.
>>>       
>> There is more logic than just KVM_SET_MSRS, such as BANK reporting
>> disabling, overwriting rules, triple fault for UC MCE during MCIP.
>> Although these logic can be implemented in user space, I think put them
>> in kernel space is easy to be understood. And the code is pretty short.
>>     
>
> IMO the main reason to put this in kernel-space would be to make it
> possible to automatically forward some MCE errors generated by the
> real hardware (RAM ECC errors for example) down into the VM.  Right
> now I suppose you could do that with the patches to forward RAM-based
> hard MCEs to userspace using SIGSEGV and handling the SIGSEGV in
> userspace, but that seems more fragile to me.
>   

FWIW: I would be looking for a way to generate MCE to report async vbus
faults via my SHM_SIGNAL_FAULT() mechanism, so I am very much in favor
of this support being in-kernel.

-Greg

> Cheers,
> Kyle Moffett
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