linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

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Message-ID: <4A018DF2.6010301@novell.com>
Date:	Wed, 06 May 2009 09:17:38 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Chris Wright <chrisw@...s-sol.org>
CC:	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

Chris Wright wrote:
> * Gregory Haskins (ghaskins@...ell.com) wrote:
>   
>> Chris Wright wrote:
>>     
>>> But a free-form hypercall(unsigned long nr, unsigned long *args, size_t count)
>>> means hypercall number and arg list must be the same in order for code
>>> to call hypercall() in a hypervisor agnostic way.
>>>       
>> Yes, and that is exactly the intention.  I think its perhaps the point
>> you are missing.
>>     
>
> Yes, I was reading this as purely any hypercall, but it seems a bit
> more like:
>  pv_io_ops->iomap()
>  pv_io_ops->ioread()
>  pv_io_ops->iowrite()
>   

Right.

> <snip>
>   
>> Today, there is no equivelent of a platform agnostic "iowrite32()" for
>> hypercalls so the driver would look like the pseudocode above except
>> substitute with kvm_hypercall(), lguest_hypercall(), etc.  The proposal
>> is to allow the hypervisor to assign a dynamic vector to resources in
>> the backend and convey this vector to the guest (such as in PCI
>> config-space as mentioned in my example use-case).  The provides the
>> "address negotiation" function that would normally be done for something
>> like a pio port-address.   The hypervisor agnostic driver can then use
>> this globally recognized address-token coupled with other device-private
>> ABI parameters to communicate with the device.  This can all occur
>> without the core hypervisor needing to understand the details beyond the
>> addressing.
>>     
>
> VF drivers can also have this issue (and typically use mmio).
> I at least have a better idea what your proposal is, thanks for
> explanation.  Are you able to demonstrate concrete benefit with it yet
> (improved latency numbers for example)?
>   

I had a test-harness/numbers for this kind of thing, but its a bit
crufty since its from ~1.5 years ago.  I will dig it up, update it, and
generate/post new numbers.

Thanks Chris,
-Greg



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