[edk2-devel] [PATCH] UefiCpuPkg: Fix SMM code hangs when InitPaging

[Gerd Hoffmann]

On Wed, Jan 18, 2023 at 01:13:43AM +0000, Zhiguang Liu wrote:
> Thanks all for reviewing, and I will send a new version to address the comment.
> 
> As for Gerd's question, let me explain.
> Let's see one example, that the CPU has SizeOfMemorySpace >48, but the CPU doesn't enable 5 level paging.
> The purpose of the current function InitPaging is to modify existing
> page table. To use the same logic to handle both 5 level and 4 level
> paging, for 4 level paging, the logic will create a false 5 level
> paging entry to treat it like a 5 level page table.

Yes.  Same for 3-level paging btw.  There are always page tables for 5
levels, but the higher levels might be unused.

> This way, the
> number of 5 level paging should always be one. If we use
> SizeOfMemorySpace to calculate the 5 level paging entry count, we will
> get number more than one.  However, as I just mentioned, we only
> create one false 5 level paging entry, system may hang when we try to
> access the second 5 level paging entry.

If 5-level paging is turned off the CPU should not see what you are
doing with the page tables for the second (and higher) 5-level entry.

So, limiting the number of 5-level entries does make sense.  Higher
entries are not used, so it's pointless work.

But that doesn't answer the question:  Why does that fix the system
hanging?  I just can't see a reason for that when looking through the
InitPaging code.  I suspect this might hide a bug somewhere else.

Related:  We got UefiCpuPkg/Library/CpuPageTableLib last year, can this
be used instead?

take care,
  Gerd



-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#98777): https://edk2.groups.io/g/devel/message/98777
Mute This Topic: https://groups.io/mt/96045489/1813853
Group Owner: devel+owner at edk2.groups.io
Unsubscribe: https://edk2.groups.io/g/devel/unsub [edk2-devel-archive at redhat.com]
-=-=-=-=-=-=-=-=-=-=-=-