linux-kernel - Re: [PATCH 01/16] KVM-HDR: register KVM basic header infrastructure

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Message-ID: <4D405853.6010003@codemonkey.ws>
Date:	Wed, 26 Jan 2011 11:22:27 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Glauber Costa <glommer@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, aliguori@...ibm.com
Subject: Re: [PATCH 01/16] KVM-HDR: register KVM basic header infrastructure

On 01/26/2011 09:36 AM, Glauber Costa wrote:
> On Wed, 2011-01-26 at 17:12 +0200, Avi Kivity wrote:
>    
>> On 01/26/2011 02:13 PM, Glauber Costa wrote:
>>      
>>>>   - it doesn't lend itself will to live migration.  Extra state must be
>>>>   maintained in the hypervisor.
>>>>          
>>> Yes, but can be queried at any time as well. I don't do it in this
>>> patch, but this is explicitly mentioned in my TODO.
>>>        
>> Using the existing method (MSRs) takes care of this, which reduces churn.
>>      
> No, it doesn't.
>
> First, we have to explicitly list some msrs for save/restore in
> userspace anyway. But also, the MSRs only holds values. For the case I'm
> trying to hit here, being: msrs being used to register something, like
> kvmclock, there is usually accompanying code as well.
>
>
>    
>>>>   - it isn't how normal hardware operates
>>>>          
>>> Since we're trying to go for guest cooperation here, I don't really see
>>> a need to stay close to hardware here.
>>>        
>> For Linux there is not much difference, since we can easily adapt it.
>> But we don't know the impact on other guests, and we can't refactor
>> them.  Staying close to precedent means it will be easier for other
>> guests to work with a kvm host, if they choose.
>>      
> I honestly don't see the difference. I am not proposing anything
> terribly different, in the end, for the sake of this specific point of
> guest supportability it's all 1 msr+cpuid vs n msr+cpuid.
>    

If type becomes implied based on the MSR number, you'd get the best of 
both worlds, no?

I do think advertising features in CPUID is nicer than writing to an MSR 
and then checking for an ack in the memory region.

>>> * This mechanism just bumps us out to userspace if we can't handle a
>>> request. As such, it allows for pure guest kernel ->   userspace
>>> communication, that can be used, for instance, to emulate new features
>>> in older hypervisors one does not want to change. BTW, maybe there is
>>> value in exiting to userspace even if we stick to the
>>> one-msr-per-feature approach?
>>>        
>> Yes.
>>
>> I'm not 100% happy with emulating MSRs in userspace, but we can think
>> about a mechanism that allows userspace to designate certain MSRs as
>> handled by userspace.
>>
>> Before we do that I'd like to see what fraction of MSRs can be usefully
>> emulated in userspace (beyond those that just store a value and ignore it).
>>      
> None of the existing. But for instance, I was discussing this issue with
> anthony a while ago, and he thinks that in order to completely avoid
> bogus softlockups, qemu/userspace, which is the entity here that knows
> when it has stopped (think ctrl+Z or stop + cont, save/restore, etc),
> could notify this to the guest kernel directly through a shared variable
> like this.
>
> See, this is not about "new features", but rather, about between pieces
> of memory. So what I'm doing in the end is just generalizing "an MSR for
> shared memory", instead of one new MSR for each piece of data.
>    

I do think having a standard mechanism for small regions of shared 
memory between the hypervisor and guest is a reasonable thing to do.

Regards,

Anthony Liguori

> Maybe I was unfortunate to mention async_pf in the description to begin
> with.
>
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>    

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