Re: Storing Non volatile variables on SD/NAND

[Andrew Fish <afish@apple.com>]

> On Sep 19, 2017, at 9:27 PM, Udit Kumar <udit.kumar@nxp.com> wrote:
> 
> 
>> On 18 September 2017 at 22:28, Udit Kumar <udit.kumar@nxp.com> wrote:
>>> Thanks Vladimir,
>>> With your design, you did delayed write to eMMC due to sharing with
>>> OS.  But it works for you:) Say if eMMC controllers offers you a
>>> status bit, if eMMC storage is being used for not. Then this could be possible to
>> update at run time, both OS/UEFI needs to check and wait if controller is being
>> used.
>> 
>> That is the problem right there. The nice thing about a firmware spec is that you
>> don't have to care about how it was implemented if you adhere to the API rules.
> 
> Yup, we are fine as long as long UEFI firmware is stored on dedicated media. 
> 
>> Imposing additional restrictions (such as requiring the OS to be careful about not
>> using the eMMC when it may be in use by the firmware) defeats the purpose of
>> using UEFI, since you won't be able to use a generic OS anyway.
>> 
> 
> Hmm,  so far, I haven't come across where UEFI specs says, we need a separate
> Storage for firmware. (May be I missed some part of specs)
> Irrespective of storage media, we have this problem if OS and UEFI shares same  
> storage. 
> 

Udit,

Can you point out the spec that states you can't boot Linux and Windows at the same time on a PC? :)

When you write a spec it is not practical do document what is not possible, you can only document the API the rest is implied by the implementation. So for example the UEFI spec does not document why the firmware and OS can't share a hardware device, just like you can't have 2 operating systems running on bare metal at the same time. It is a little like Occam's Razor the reason that the firmware and the OS can not share a hardware device is the mechanics of how to share a hardware device is not defined in the spec, thus it is not part of the API and not possible. 

Thanks,

Andrew Fish

>>> For sure,  some synchronization issues need to be ironed out (or maybe I am
>> just dreaming here).
>>> 
>>> On part 2) where you forked VariableRuntime driver , could we think of
>>> updating VariableRuntime driver, to support non-XIP or memory mapped
>> devices.
>>> 
>> 
>> I think being able to support non-memorymapped FV volumes for the variable
>> store would be a big improvement. This does require changes to both the
>> FaultTolerantWrite drivers and the VariableRuntime drivers, which both appear
>> in PEI, DXE and SMM flavors, and require thorough review due to the security
>> impact bugs have in this layer, so this is a rather large chunk of work to take on.
> 
> Thanks,  your list is longer than what I was thinking :-)
> I think, for embedded world with UEFI, later or sooner, this will be required.
> 
> Thanks
> Udit
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