Re: [edk2-devel] [PATCH v1 1/1] StandaloneMmPkg: Initialise serial port early in StandaloneMmEntryPoint

["levi.yun" <yeoreum.yun@arm.com>]

Hi, Laszlo.
> This will keep initing the serial port upon every API call until the
> global variable becomes writeable, and then the next API call will init
> the serial port for one last time, and also prevent further page table
> checks.
>
> The CheckWritable() function is an implementation detail. In the DXE
> phase, it could be implemented (I think?) with the
> GetMemorySpaceDescriptor() DXE service, or perhaps even the
> EFI_MEMORY_ATTRIBUTE_PROTOCOL.GetMemoryAttributes() UEFI protocol member
> function. In the standalone MM core, CheckWritable() could walk the page
> tables explicitly. The idea is, either way, to *predict* whether writing
> to "mInitialized" would trap.
I think it wouldn't proper, to DxeCore and StMM too.
IIUC,  before CoreInitializeMemoryServices is called, we couldn't use
that method in case DxeCore.
And the problem now I face is also StMM before populating memory
information (done in LibConstructor).


> Now I think that speculative / out of order execution could actually
> trigger the trap *before* GlobalsWriteable is calculated; however, I
> think such a trap should be architecturally hidden (i.e., invisible). I
> think at worst we could need a compiler barrier (maybe throw in some
> "volatile" for GlobalsWriteable and mInitialized), so that the
> *compiler* not try to reorder the accesses. But even that sounds like a
> stretch.
Agree if we develop CheckPerm??

Currently, (In my narrow thinking) there is no more generic solution
than create new interface SerialPortInitilizeEarly.

Am I missing?

Many Thanks.

--------
Sincerely,
Levi.
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