From mboxrd@z Thu Jan 1 00:00:00 1970 From: Julien Grall Subject: Re: Design doc of adding ACPI support for arm64 on Xen - version 3 Date: Tue, 18 Aug 2015 12:01:10 -0700 Message-ID: <55D380F6.60700@citrix.com> References: <55CE0247.4030805@linaro.org> <55D21AE2020000780009B6D4@prv-mh.provo.novell.com> <55D2099F.2000903@citrix.com> <55D2CC5F02000078000D6BC5@prv-mh.provo.novell.com> <55D2D3F6.1030405@citrix.com> <55D2F472020000780009B8CC@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55D2F472020000780009B8CC@prv-mh.provo.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: hangaohuai@huawei.com, ian.campbell@citrix.com, stefano.stabellini@eu.citrix.com, andrew@fubar.geek.nz, peter.huangpeng@huawei.com, stefano.stabellini@citrix.com, david.vrabel@citrix.com, zhaoshenglong@huawei.com, boris.ostrovsky@oracle.com, xen-devel@lists.xen.org, parth.dixit@linaro.org, christoffer.dall@linaro.org, shannon.zhao@linaro.org List-Id: xen-devel@lists.xenproject.org On 18/08/2015 00:01, Jan Beulich wrote: >>>> On 18.08.15 at 08:43, wrote: >> Hi Jan, >> >> On 17/08/2015 22:10, Jan Beulich wrote: >>>>>> Julien Grall 08/17/15 6:27 PM >>> >>>> On 17/08/2015 08:33, Jan Beulich wrote: >>>>>>>> On 14.08.15 at 16:59, wrote: >>>>>> b) Create EFI_MEMORY_DESCRIPTOR table. This will add memory start and >>>>>> size information of Dom0. And Dom0 will get the memory information >>>>>> through this EFI table. >>>>> >>>>> To some degree the same applies here: While I see that you have no >>>>> legacy vehicle like x86's E820, I also don't see how Dom0 - not being >>>>> able to make EFI boot or runtime services calls - would get hold of this >>>>> table. And if a non-EFI mechanism is to be used here, using the EFI >>>>> data structure would turn out to be just an arbitrary (or convenience) >>>>> decision, not something inherently required. Which I think should be >>>>> said explicitly if so, rather than leaving the reader guess. >>>> >>>> It's not an arbitrary decision, when UEFI stub in Linux is using device >>>> tree properties to pass the UEFI table to the kernel ([1]). >>>> >>>> When booting on Xen with ACPI, dom0 will use the non-EFI entry point. >>>> The easiest way to pass the memory information to Linux is using the >>>> UEFI DT properties. >>> >>> In which case it is even more arbitrary to use the EFI data structure to >>> convey memory information (instead of expressing it in plain DT, which is >>> how I blindly assume non-EFI does it). Of course there's the small chance >>> that "UEFI DT properties" implies a certain binary format, but it's still >> odd >>> for a non-EFI entry point to assume EFI properties to be there... >> >> Linux is able to boot either on ACPI or DT. When ACPI is used, the EFI >> stub (mandatory) will create a small DT in order to pass the command >> line and other informations (such as the EFI memory table) to the >> kernel. This is because the stub is self-contained and and never use >> variable living in the kernel. >> >> In order to know whether you are booting using DT or ACPI, they check if >> the DT contains only the /chosen node. >> >> Actually the EFI-stub will always jumped on the non-EFI path. >> >> By passing the minimal DT as suggested on the design doc, we are >> avoiding to get a different code path for Xen in the kernel and we are >> finally acting as the EFI-stub was a nop for DOM0. >> >> So I don't see how this would be arbitrary... > > Indeed with your explanation this doesn't look arbitrary anymore. > But none of this was spelled out in the original document. Sorry I'm forgetting every time that you may not be aware of all the justifications. Regards, -- Julien Grall