[PATCH v2 2/4] OvmfPkg/Sec/Ia32: seed the temporary RAM with PcdInitValueInTempStack

[Laszlo Ersek <lersek@redhat.com>]

This allows the PEI core to report the maximum temporary SEC/PEI stack
usage on the DEBUG_INFO level, in the PeiCheckAndSwitchStack() function
[MdeModulePkg/Core/Pei/Dispatcher/Dispatcher.c]:

* Normal boot:

> Temp Stack : BaseAddress=0x814000 Length=0x4000
> Temp Heap  : BaseAddress=0x810000 Length=0x4000
> Total temporary memory:    32768 bytes.
>   temporary memory stack ever used:       3664 bytes. <----
>   temporary memory heap used for HobList: 5904 bytes.
>   temporary memory heap occupied by memory pages: 0 bytes.

* S3 resume (with PEI decompression / SMM):

> Temp Stack : BaseAddress=0x814000 Length=0x4000
> Temp Heap  : BaseAddress=0x810000 Length=0x4000
> Total temporary memory:    32768 bytes.
>   temporary memory stack ever used:       3428 bytes. <----
>   temporary memory heap used for HobList: 4816 bytes.
>   temporary memory heap occupied by memory pages: 0 bytes.

I unit-tested this change by transitorily adding an infinite loop right
after the "rep stosd", and dumping the guest's temp SEC/PEI RAM (32KB
currently) while the guest was stuck in the loop. The dump includes one
dword from before and after the temp SEC/PEI RAM:

> $ virsh qemu-monitor-command GUEST_NAME --hmp 'xp /8194wx 0x80FFFC'
>
> 000000000080fffc: 0x00000000 0x5aa55aa5 0x5aa55aa5 0x5aa55aa5
> 000000000081000c: 0x5aa55aa5 0x5aa55aa5 0x5aa55aa5 0x5aa55aa5
> ...
> 0000000000817fec: 0x5aa55aa5 0x5aa55aa5 0x5aa55aa5 0x5aa55aa5
> 0000000000817ffc: 0x5aa55aa5 0x00000000

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=747
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---

Notes:
    v2:
    - update comment on ES [Jordan]
    - document the other segment registers too [Jordan]
    - move seeding to top of routine [Jordan]
    - hence, drop earlier EAX -> EBX renaming [Jordan]
    - replace runtime right shift with compile-time division [Jordan]
    - drop Ard's Reviewed-by

 OvmfPkg/Sec/SecMain.inf        |  1 +
 OvmfPkg/Sec/Ia32/SecEntry.nasm | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/OvmfPkg/Sec/SecMain.inf b/OvmfPkg/Sec/SecMain.inf
index 711b59530907..6051cb3c6c4c 100644
--- a/OvmfPkg/Sec/SecMain.inf
+++ b/OvmfPkg/Sec/SecMain.inf
@@ -71,6 +71,7 @@ [Pcd]
   gEfiMdePkgTokenSpaceGuid.PcdGuidedExtractHandlerTableAddress
   gUefiOvmfPkgTokenSpaceGuid.PcdGuidedExtractHandlerTableSize
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDecompressionScratchEnd
+  gEfiMdeModulePkgTokenSpaceGuid.PcdInitValueInTempStack
 
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
diff --git a/OvmfPkg/Sec/Ia32/SecEntry.nasm b/OvmfPkg/Sec/Ia32/SecEntry.nasm
index 7fee1c2b2e4f..03501969ebce 100644
--- a/OvmfPkg/Sec/Ia32/SecEntry.nasm
+++ b/OvmfPkg/Sec/Ia32/SecEntry.nasm
@@ -29,12 +29,30 @@ extern ASM_PFX(SecCoreStartupWithStack)
 ; @param[in]  EAX   Initial value of the EAX register (BIST: Built-in Self Test)
 ; @param[in]  DI    'BP': boot-strap processor, or 'AP': application processor
 ; @param[in]  EBP   Pointer to the start of the Boot Firmware Volume
+; @param[in]  DS    Selector allowing flat access to all addresses
+; @param[in]  ES    Selector allowing flat access to all addresses
+; @param[in]  FS    Selector allowing flat access to all addresses
+; @param[in]  GS    Selector allowing flat access to all addresses
+; @param[in]  SS    Selector allowing flat access to all addresses
 ;
 ; @return     None  This routine does not return
 ;
 global ASM_PFX(_ModuleEntryPoint)
 ASM_PFX(_ModuleEntryPoint):
 
+    ;
+    ; Fill the temporary RAM with the initial stack value.
+    ; The loop below will seed the heap as well, but that's harmless.
+    ;
+    mov     eax, FixedPcdGet32 (PcdInitValueInTempStack)      ; dword to store
+    mov     edi, FixedPcdGet32 (PcdOvmfSecPeiTempRamBase)     ; base address,
+                                                              ;   relative to
+                                                              ;   ES
+    mov     ecx, FixedPcdGet32 (PcdOvmfSecPeiTempRamSize) / 4 ; dword count
+    cld                                                       ; store from base
+                                                              ;   up
+    rep stosd
+
     ;
     ; Load temporary RAM stack based on PCDs
     ;
-- 
2.14.1.3.gb7cf6e02401b