linux-kernel - Re: [PATCH] Add MCE support to KVM

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Date:	Mon, 20 Apr 2009 14:20:55 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	Huang Ying <ying.huang@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] Add MCE support to KVM

On 04/20/09 13:27, Avi Kivity wrote:
> Gerd Hoffmann wrote:
>>>> Well, xenner doesn't do vmcalls, so the page isn't vendor specific.
>>>
>>> Well, for true pv (not pv-on-hvm) it wouldn't use the MSR, would it?
>>
>> Yes, the MSR is used for pv-on-hvm only.
>
> So it isn't relevant for Xenner?

It is.  I still plan to merge xenner into qemu, and also support 
xenstyle pv-on-hvm drivers.

> That said, I'd like to be able to emulate the Xen HVM hypercalls. But in
> any case, they hypercall implementation has to be in the kernel,

No.  With Xenner the xen hypercall emulation code lives in guest address 
space.

> so I
> don't see why the MSR shouldn't be.

I don't care that much, but /me thinks it would be easier to handle in 
userspace ...

> Especially if we need to support
> tricky bits like continuations.

Is there any reason to?  I *think* xen does it for better scheduling 
latency.  But with xen emulation sitting in guest address space we can 
schedule the guest at will anyway.

>> Same MSR, multiple writes (page number in the low bits).
>
> Nasty. The hypervisor has to remember all of the pages, so it can update
> them for live migration.

Xenner doesn't need update-on-migration, so there is no need at all to 
remember this.  At the end of the day it is just memcpy(guest, data, 
PAGESIZE) triggered by wrmsr.

cheers,
   Gerd
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