On 3/19/20 12:31 PM, François Ozog wrote:
On Thu, 19 Mar 2020 at 12:05, Leif Lindholm <leif@nuviainc.com mailto:leif@nuviainc.com> wrote:
Hi Ilias, On Thu, Mar 19, 2020 at 12:33:30 +0200, Ilias Apalodimas wrote: > On Thu, Mar 19, 2020 at 10:45:31AM +0100, Heinrich Schuchardt wrote: > > > > As some of you may know, I'm going to submit a RFC patch series for UEFI > > > > capsule support on U-Boot in a week or so (maybe). With this patch, > > > > we will support "Capsule-on-Disk" only (but without authentication > > > > implemented). > > > > > > > > One of my concerns here is about "variable update via a capsule" as > > > > an alternative of "SetVariable at runtime," where values to be set > > > > for variables will be exported as a capsule file and all the updates > > > > will take place after rebooting. This is very useful on systems > > > > where SetVariable is not available at runtime for some reasons. > > > > Some idea has been proposed by Peter[1], but the discussion has been > > > > stalled for a long time. > > > > > > > > [1] https://lists.linaro.org/pipermail/boot-architecture/2018-October/000883.html > > > > > > So the way i understand it we got two paths we can follow: > > > 1. Use a configuration table for those variables > > > 2. Shadow the variables on a kernel accessible memory in order to have access to > > > them > > > > It is unclear to me why it would be advantageous to follow any of these > > ideas compared to using GetVariable and SetVariable and letting the > > firmware runtime manage the memory area. > > This describes some of the reasoning behind the idea. > https://lists.linaro.org/pipermail/boot-architecture/2018-September/000841.html > The short version is that we should be able to provide GetVariable() even if > SetVariable() is not implemented. I'm pretty sure Heinrich's concerns with this description are similar to my own, and boil down to one thing: this sounds like inventing new interfaces for doing these operations on these systems. Systems that support GetVariable need to support that without any special handling added to the kernel. For systems that have the flash sharing issue, this will mean that U-Boot needs to copy the flash variable region into RAM at ExitBootServices() time, and have the runtime service use this area. Similarly, for such systems to support SetVariable they will need to either: - allocate some spare space for the GetVariable buffer and rewrite the whole thing on each write. - allocate a separate memory region to accept write transactions into, and hook those transactions into the GetVariable lookup to remain consistent. (The former isn't necessarily going to be less effort, and the latter is almost certainly going to be required at some point.) How and when any transactions become persisted in flash are separate questions.I haven't followed the technical discussion. following is the original intent behind the changes requested for variable handling:
- runtime services are not the best things and even EDK2 are probably
moving away from this (Grant told us that at a design sprint a year ago)
Operating systems like Windows rely on setting Boot* variables. Do you plan to break them?
- "system administrator" is not the trusted party that has authority to
change a variable, a "platform administrator" has the authority and is the signing party of variable updates.
Not all variables are signed. Especially Boot* variables are managed by users with access to efibootmgr.
- UEFI variables interface for tools/kernel must not change: get/set
variable
- implementation of getVariable accesses a copy/shadow version of the
UEFI variable
- implementation of setVariable actually generates an update capsule so
that the variable update is seen at next reboot
The discussion as far as I understood it centered on the question how a firmware that has no access to persistent memory at runtime can handle non-volatile variable updates.
A capsule is only a possible encoding of information but does not answer the question in which medium the information from before the reboot shall be conveyed to after the reboot.
Putting the information into RAM and hoping it survives a reboot is feasible but will not work during a system shutdown. Letting the operating system write the information to disk seems to be the easiest thing to do.
Best regards
Heinrich