> -----Original Message----- > From: Heinrich Schuchardt <xypron.g...@gmx.de> > Sent: Monday, March 1, 2021 2:39 PM > To: Ilias Apalodimas <ilias.apalodi...@linaro.org>; Grant Likely > <grant.lik...@arm.com> > Cc: Boot Architecture Mailman List <boot-architecture@lists.linaro.org>; Samer > El-Haj-Mahmoud <samer.el-haj-mahm...@arm.com>; Ard Biesheuvel > <a...@kernel.org>; Leif Lindholm <l...@nuviainc.com> > Subject: Re: EFI_LOAD_FILE2 for initrd standardization
... > > The UEFI spec knows two types of separators for device paths. Both have type > 0x7F (End of Hardware Device Path) but differ by the sub-type: > > Sub-Type 0xff – End Entire Device Path > Sub-Type 0x01 – End Instance of a Device Path > > Field EFI_LOAD_OPTION.FilePathList[] is described in the UEFI spec as > follows: > > "A packed array of UEFI device paths. The first element of the array is a > device > path that describes the device and location of the Image for this load > option." > > It is not immediately clear if the separators between the array elements are > of > sub-type 0xff or 0x01. The description in the UEFI spec should be reworked for > more clarity. > Agree that this is not clear, and could be interpreted either way. And yes, agree the UEFI spec needs a clarification > The current EDK II coding requires that the device path identifying the UEFI > binary (i.e. FilePathList[0] is terminated by a sub-type 0xff end node. > The EDK2 code seems to be incomplete, with a "TODO" to support the FilePathList[]. In fact, the code calls it "FilePath" to be clear that it is assuming a single DevicePath (which means a 0xFF sub-type termination) https://github.com/tianocore/edk2/blob/master/MdeModulePkg%2FLibrary%2FUefiBootManagerLib%2FBmLoadOption.c#L199 Searching the code further, I see there is support for parsing multi-instance device path (separated by END_INSTANCE_DEVICE_PATH_SUBTYPE, or 0x1) in things like UefiDevicePathLib , parsing code, etc... But it does not seem to be supported at all in the UEFI Boot Manager implemented in EDK2 > The UEFI variable ConDev is decribed as "The device path of all possible > console > input devices". The spec does not refer to it as an array of device paths. > Looking at EDK2, ConXDev are treated as multi-instance devices, with ConPlatformDxe using AppendDevicePathInstance() and GetNextDevicePathInstance() to construct / navigate the multiple-instances The spec should have defined these as a "multi-instance device path of all possible console X devices". See my notes below on multi-instance DP. > So it seems that the UEFI spec editors mean by array of device paths that an > element of the array is separated by sub-type 0xff. Each individual array > element > may be a collection of device paths instances separated by 0x01 sub-type end > nodes. > I am leaning towards this conclusion as well. This is further supported by the following evidence: * EFI_DEVICE_PATH_UTILITIES_PROTOCOL has a function called IsDevicePathMultiInstance() along with walker functions AppendDevicePathInstance(), GetNextDevicePathInstance(). The description of these functions make it clear that the intention is to treat 0x01 as a separator between multiple instances of device paths in a multi-instance device path structure, with 0xff as the final end of DP indicator. For example, AppendDevicePathInstance() is defined as: " This function creates a new device path by appending a copy of the specified device path instance to a copy of the specified device path in an allocated buffer. The end-of-device-path device node is moved after the end of the appended device node and a new end-of-device-path-instance node is inserted between." * In EFI_USER_INFO , there are user info policy types that leverage the device path definitions (such as EFI_USER_INFO_ACCESS_FORBID_LOAD), and they are clear on distinguishing the term "multi-instance device path" from a "series of UEFI device paths": "The record is a series of normal UEFI device paths (not multi-instance device paths)." * The "multi-instance" usage also shows up in the language of LocateDevicePath() So if the spec intended for FilePathList[] to use the 0x1 separator, they should have used the term "multi-instance" device path, which is not the case. > In the device path spec a sentence could be added as follows: > > "A packed array of UEFI device paths. The first element of the array is a > device > path that describes the device and location of the Image for this load option. > *Each array element is terminated by a sub-type 0xff, End Entire Device Path > node.*" > Yes. Or could simply say: "A packed array of UEFI device paths (not multi-instance device paths). The first element of the array is a device path that describes the device and location of the Image for this load option." Either way, we can do a "code first" ECR in the UEFI spec. I say code first to ensure that it remains in public domain, and not blocked by UEFI Forum NDA until the publication of the next version of the UEFI spec. If we agree, I can get this process started and propose the language. > Adding initrd or device tree device paths could be implemented as follows: > > Array element [0]: > device path of the binary (e.g. the Linux kernel) terminated by 0xff. > > Array element [i], i > 0: > device paths of the different initial RAM disks separated by 0x01 instance end > nodes and terminated by an 0xff entire path node. > > To identify the device paths with all its instances as initrds we can prepend > a > VenMedia() device path node with a specific GUID to the entire path. > > Array element [j], j > 0: > device path of the device-tree possibly followed by instances of device paths > of > device-tree overlays separated by 0x01 instance end nodes and terminated by an > 0xff entire path node. > > To identify the device paths with all its instances as device trees and > device tree > overlays we can prepend a VenMedia() device path node with a specific GUID to > the entire path. > One suggestion here is to take into consideration: whether we standardize this proposal (for loading additional kernel files from element[i], i>0 ) or not, the FW implementation must still be compliant with the UEFI rules around booting from a "short form device path" (UEFI spec, section 3.1.2) > Best regards > > Heinrich IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. 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