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Subject: Re: [PATCH 18/19] KVM: PPC: Book3S HV: add passthrough support
To: Paul Mackerras <paulus@ozlabs.org>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>
References: <20190107184331.8429-1-clg@kaod.org>
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 <20190122052657.GG15124@blackberry>
 <b3788899d86b1d3bb8d141f1126592e3593ac8ef.camel@kernel.crashing.org>
 <20190123103009.GB29826@blackberry>
From: =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>
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Date: Wed, 23 Jan 2019 12:07:19 +0100
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Cc: linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
 David Gibson <david@gibson.dropbear.id.au>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 1/23/19 11:30 AM, Paul Mackerras wrote:
> On Wed, Jan 23, 2019 at 05:45:24PM +1100, Benjamin Herrenschmidt wrote:
>> On Tue, 2019-01-22 at 16:26 +1100, Paul Mackerras wrote:
>>> On Mon, Jan 07, 2019 at 08:10:05PM +0100, Cédric Le Goater wrote:
>>>> Clear the ESB pages from the VMA of the IRQ being pass through to the
>>>> guest and let the fault handler repopulate the VMA when the ESB pages
>>>> are accessed for an EOI or for a trigger.
>>>
>>> Why do we want to do this?
>>>
>>> I don't see any possible advantage to removing the PTEs from the
>>> userspace mapping.  You'll need to explain further.
>>
>> Afaik bcs we change the mapping to point to the real HW irq ESB page
>> instead of the "IPI" that was there at VM init time.

yes exactly. You need to clean up the pages each time.
 
> So that makes it sound like there is a whole lot going on that hasn't
> even been hinted at in the patch descriptions...  It sounds like we
> need a good description of how all this works and fits together
> somewhere under Documentation/.

OK. I have started doing so for the models merged in QEMU but not yet 
for KVM. I will work on it.

> In any case we need much more informative patch descriptions.  I
> realize that it's all currently in Cedric's head, but I bet that in
> two or three years' time when we come to try to debug something, it
> won't be in anyone's head...

I agree. 


So, storing the ESB VMA under the KVM device is not shocking anyone ?  

Thanks,

C.