linux-kernel - Re: [PATCH v4 14/24] xen/arm: initialize grant

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Date:	Fri, 14 Sep 2012 16:08:44 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Tim (Xen.org)" <tim@....org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 14/24] xen/arm: initialize grant_table on ARM

On Fri, 2012-09-14 at 15:56 +0100, Stefano Stabellini wrote:
> On Fri, 14 Sep 2012, Konrad Rzeszutek Wilk wrote:
> > On Fri, Sep 14, 2012 at 12:13:16PM +0100, Stefano Stabellini wrote:
> > > Initialize the grant table mapping at the address specified at index 0
> > > in the DT under the /xen node.
> > > After the grant table is initialized, call xenbus_probe (if not dom0).
> > 
> > So we don't really care about the grant's size then? The DT xen.txt
> > talks about it..
> 
> I am assuming that the size of the memory region specified in the device
> tree is sufficiently large to map the entire grant table, given that both
> the device tree hypervisor entry and the grant table size comes from Xen.

Actually, the grant table can grow dynamically under the control of the
guest, I think you just pass GNTTABOP_setup_table with some more frames.
See drivers/xen/grant_table.c:gnttab_expand().

> The grant table size is currently queried to Xen directly via an
> hypercall (GNTTABOP_query_size). Basically the size in the device tree
> is redundant information.

This size is the size of the physical address space where the guest
could chose map grant table frames. It could be either larger or smaller
than the actual grant table. (smaller because the guest could use
physical addresses not within this region, if it wanted to for some
reason).

Ian.

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