linux-kernel - Re: [PATCH v2] tile: support KVM for tilegx

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Message-ID: <521E9531.904@tilera.com>
Date:	Wed, 28 Aug 2013 20:26:25 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>
Subject: Re: [PATCH v2] tile: support KVM for tilegx

On 8/26/2013 8:04 AM, Gleb Natapov wrote:
> On Sun, Aug 25, 2013 at 09:26:47PM -0400, Chris Metcalf wrote:
>> On 8/25/2013 7:39 AM, Gleb Natapov wrote:
>>> On Mon, Aug 12, 2013 at 04:24:11PM -0400, Chris Metcalf wrote:
>>>> This change provides the initial framework support for KVM on tilegx.
>>>> Basic virtual disk and networking is supported.
>>>>
>>> This needs to be broken down to more reviewable patches.
>> I already broke out one pre-requisite patch that wasn't strictly KVM-related:
>>
>> https://lkml.org/lkml/2013/8/12/339
>>
>> In addition, we've separately arranged to support booting our kernels in a way that is compatible with the Tilera booter running at the highest privilege level, which enables multiple kernel privilege levels:
>>
>> https://lkml.org/lkml/2013/5/2/468
>>
>> How would you recommend further breaking down this patch?  It's pretty much just the basic support for minimal KVM.  I suppose I could break out all the I/O related stuff into a separate patch, though it wouldn't amount to much; perhaps the console could also be broken out separately.  Any other suggestions?
>>
> First of all please break out host and guest bits. Also I/O related stuff,
> like you suggest (so that guest PV bits will be in separate patch) and
> change to a common code (not much as far as I see) with explanation why
> it is needed. (Why kvm_vcpu_kick() is not needed for instance?)

I broke it down into three pieces in the end: the basic host support, the basic guest PV support, and the virtio/console support.  The first piece is still much the biggest.  I found that the generic kvm_vcpu_kick() is fine, so I removed the custom version (which predated the generic version in our internal tree).  Explanations are now in the git commit comments.

>>> Also can you
>>> describe the implementation a little bit? Does tile arch has vitalization
>>> extension this implementation uses, or is it trap and emulate approach?
>>> If later does it run unmodified guest kernels? What userspace are you
>>> using with this implementation?
>> We could do full virtualization via trap and emulate, but we've elected to do a para-virtualized approach.  Userspace runs at PL (privilege level) 0, the guest kernel runs at PL1, and the host runs at PL2.  We have available per-PL resources for various things, and take advantage of having two on-chip timers (for example) to handle timing for the host and guest kernels.  We run the same userspace with either the host or the guest.
>>
> OK, thanks for explanation. Why have you decided to do PV over trap and
> emulate?

Performance and simplicity; I added comments to the git commit to provide a rationale.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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