linux-kernel - Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

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Date:	Tue, 22 Sep 2015 11:59:36 +0100
From:	Ian Campbell <ian.campbell@...rix.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	"Stefano Stabellini" <stefano.stabellini@...citrix.com>
CC:	Julien Grall <julien.grall@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <wei.liu2@...rix.com>,
	<konrad.wilk@...cle.com>, <linux-kernel@...r.kernel.org>,
	<david.vrabel@...rix.com>, <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

On Mon, 2015-09-14 at 17:23 +0200, Roger Pau Monné wrote:
> I'm not saying that we shouldn't take those patches, I'm just saying
> that IMHO this is a workaround, and I would like to see a plan and
> somebody committed to have it fixed in a proper way, by introducing a
> 64KB PV block protocol.

In my view the basic unit of operation for all Xen interfaces (on x86 and
arm at least[0]) is 4K. The peer at either end of a PV protocol should
therefore be entitled to assume that at a minimum the other end supports
operation in 4K mode (i.e. 4K is the baseline).

Operations in larger sizes (which would necessarily be multiples of 4K) are
then an extension which is subject to a negotiation between the two ends,
it doesn't really matter if those larger sizes arise because of the use of
superpages or because the guest is internally using some larger basic page
size (which ARM calls a "granule", and where 64K comes from here).

I think this line of reasoning applies just as strongly to the hypercall
ABI as well BTW, they all use 4K as their basic unit, but might support
extensions to operation on multiples of that (negotiated either via a
specific error return and fallback or via the use of XENFEAT).

Ian.

[0] It's not implausible that some other architecture may exsit which
doesn't use 4K as the basic page szie)
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