Re: smm lock query

["Marvin Häuser" <Marvin.Haeuser@outlook.com>]

Hello Andrew and Laszlo,

Thanks for your comments!
Of course I'm with you that it is the platform that signals the SmmReadyToLock event and therefor is aware.
However, they might rely on the protocol's description that the resources are about(!) to be locked and code accordingly, not considering the event characteristic of the handler in PiSmmIpl.
The code might be written by different people, not especially reviewed against edk2's actions, or additional code might be supplied by third parties that do not have tree code access (which, by integration, would be "platform binaries" by the definition applying here).

Therefor I would still ask everyone to consider figuring out a solution to this discrepancy from the specification, such as the internal "dummy event" I proposed.

Thanks,
Marvin.

> -----Original Message-----
> From: Laszlo Ersek <lersek@redhat.com>
> Sent: Monday, May 28, 2018 12:58 PM
> To: Andrew Fish <afish@apple.com>; Marvin H?user
> <Marvin.Haeuser@outlook.com>
> Cc: edk2-devel@lists.01.org; Abhishek Singh <abh@cs.unc.edu>;
> ruiyu.ni@intel.com; eric.dong@intel.com; star.zeng@intel.com
> Subject: Re: [edk2] smm lock query
> 
> On 05/27/18 22:44, Andrew Fish wrote:
> >
> >
> >> On May 27, 2018, at 9:47 AM, Marvin H?user
> <Marvin.Haeuser@outlook.com> wrote:
> >>
> >> Good day Abhishek,
> >>
> >> I CC'd the MdeModulePkg maintainers, Ruiyu for the Platform BDS aspect
> (exposes the ReadyToLock protocol) and Laszlo for his high-quality answers.
> >>
> >> Strictly speaking you are, right, because of the description for the MM
> protocol:
> >> "Indicates that MM resources and services that should not be used by the
> third party code are about[Marvin: (!)] to be locked."
> >> Practically however, I don't see any issue with the current
> implementation. Code inside MMRAM is not affected directly by the lock, it is
> just notified.
> >> However, either the code or the specification should be slightly updated
> to be in sync. A code update might require review of the caller assumptions,
> just to be sure.
> >>
> >> I have a different concern though and hope I'm actually overlooking
> something.
> >> If I understand the code correctly, it is the Platform BDS that exposes the
> (S)MmReadyToLock protocol. PiSmmIpl seems to consume that event and
> lock SMM resources based on the event.
> >> Because of latter being an event however, I don't think it is, or can be,
> guaranteed to be the last event group member executing.
> >> When it is not the last, the "about to be locked" part is not true for any
> subsequent callbacks, that could actually be a risky break of the specification
> - if it is.
> >> If it is a break of the specification, I can only think of letting Platform BDS
> expose an "internal" event group, which is only caught by PiSmmIpl, which
> then drives the actual SmmReadyToLock flow.
> >> This would require updates to all platform trees and hence I would
> propose a temporary backwards-compatibility.
> >>
> >> Any comments? Did I overlook something (I hope)?
> >>
> >
> > Mavvin,
> >
> > You are correct there is no guarantee of order in events. Thanks for cc'ing
> the right folks, as I don't remember all the low level details...
> >
> > In general the idea behind the MM code is it only comes from the platform,
> then by definition that code should be aware when the platform was going
> to lock MM. In a practical sense any MM module that had a depex evaluate
> to true would have dispatched in DXE prior to BDS being launched. In general
> BDS is the code that enumerates PCI and connects devices, thus there is no
> chance for 3rd party software to run before that point in the boot. So in an
> abstract sense that lock represents the end of DXE dispatch.
> 
> This is my understanding as well. gEfiDxeSmmReadyToLockProtocolGuid is
> installed by Plaform BDS before any non-platform binaries get a chance to
> run. In terms of the current PlatformBootManagerLib interfaces, that means
> the protocol should be installed from
> PlatformBootManagerBeforeConsole() (as noted on the API declaration
> itself).
> 
> Thanks
> Laszlo