Re: [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space

From: Wei Liu <wei.liu2@citrix.com>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Steve Capper <Steve.Capper@arm.com>,
	George Dunlap <dunlapg@umich.edu>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	"Huangpeng (Peter)" <peter.huangpeng@huawei.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Shannon Zhao <shannon.zhao@linaro.org>,
	Shannon Zhao <zhaoshenglong@huawei.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space
Date: Tue, 2 Aug 2016 12:01:34 +0100	[thread overview]
Message-ID: <20160802110134.GZ22419@citrix.com> (raw)
In-Reply-To: <a4b83d46-0771-9f1d-e73a-c4f9f7168a85@arm.com>

On Thu, Jul 28, 2016 at 12:06:11PM +0100, Julien Grall wrote:
> Hi,
> 
> On 26/07/16 02:17, Boris Ostrovsky wrote:
> >On 07/25/2016 07:40 PM, Stefano Stabellini wrote:
> >>On Mon, 25 Jul 2016, Boris Ostrovsky wrote:
> >>>On 07/25/2016 06:06 PM, Stefano Stabellini wrote:
> >>>>On Mon, 25 Jul 2016, George Dunlap wrote:
> >>>>>On Thu, Jul 21, 2016 at 10:15 PM, Stefano Stabellini
> >>>>><sstabellini@kernel.org> wrote:
> >>>>Going back to the discussion about how to account for the ACPI blob in
> >>>>maxmem, let's make this simple, if we increase maxmem by the size of
> >>>>the
> >>>>ACPI blob:
> >>>>
> >>>>- the toolstack allocates more RAM than expected (bad)
> >>>>- when the admin specifies 1GB of RAM, the guest actually gets 1GB of
> >>>>   usable RAM (good)
> >>>>- things are faster as Xen and the guest can exploit superpage mappings
> >>>>   more easily at stage-1 and stage-2 (good)
> >>>>
> >>>>Let's call this option A.
> >>>>
> >>>>If we do not increase maxmem:
> >>>>
> >>>>- the toolstack allocates less RAM, closer to the size specified in the
> >>>>   VM config file (good)
> >>>>- the guest gets less usable memory than expected, less than what was
> >>>>   specified in the VM config file (bad)
> >>>
> >>>Not sure I agree with this, at least for x86/Linux: guest gets 1GB of
> >>>usable RAM and part of that RAM stores ACPI stuff. Guest is free to
> >>>stash ACPI tables somewhere else or ignore them altogether and use that
> >>>memory for whatever it wants.
> >>On ARM it will be a ROM (from guest POV)
> >
> >
> >In which case I don't see why we should take it from maxmem allocation.
> >I somehow thought that there was a choice of whether to put it in ROM or
> >RAM on ARM but if it's ROM only then I don't think there is an option.
> 
> We have option to do the both on ARM. I just feel that the ROM option is a
> cleaner interface because the ACPI tables are not supposed be modified by
> the guest, so we can prevent to be overridden (+ all the advantages
> mentioned by Stefano with option A).
> 
> >IIUIC the toolstack pretends that the blob goes to memory because that's
> >how its interfaces work but that space is not really what we think about
> >when we set memory/maxmem in the configuration file. Unlike x86.
> 
> I think we need to draw a conclusion for Shannon to continue to do the work
> and I would like to see this series in Xen 4.8. From my understanding you
> are for option B, so does George.
> 
> Stefano votes for option A, but find B acceptable. Any other opinions?
> 

From my PoV I just need things to be clearly documented.

Wei.

> Regards,
> 
> -- 
> Julien Grall

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