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OvmfPkg/PlatformInitLib: restore below-4G low memory detection for TDVF

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Commit 0a0919607c ("OvmfPkg/PlatformInitLib: redefine low memory") narrowed
PlatformGetLowMemoryCB() to consider only the first below-4G memory block
whose base address is zero. The change was intended to fix SVSM guests,
where SVSM caves a chunk out of below-4G RAM and OVMF must not stray into
that hole.

TDVF, however, reports its below-4G RAM through the TdHob as two adjacent
resource descriptors:

  [0, 0x800000)              EFI_RESOURCE_SYSTEM_MEMORY   (pre-accepted)
  [0x800000, ~4G)            EFI_RESOURCE_MEMORY_UNACCEPTED

PlatformScanE820Tdx() surfaces both as EfiAcpiAddressRangeMemory E820
entries. After 0a0919607c only the first, tiny 8 MiB block is picked up,
so PlatformInfoHob->LowMemory becomes 0x800000.
In OvmfPkg/PlatformPei/MemDetect.c PublishPeiMemory() this drives:

  LowerMemorySize = 0x00800000            // LowMemory
  PeiMemoryCap    = 0x04F82000            // ~81 MiB
  MemoryBase      = LowerMemorySize - PeiMemoryCap   // UINT32 underflow
                  = 0xFB87E000

Permanent PEI memory is then published at 0xFB87E000, which is not backed
by RAM. TemporaryRamMigration()'s first CopyMem into that phantom range
(observed as 0xFB898000 in the failing log) faults, tearing down the TD.

Fold adjacent below-4G memory blocks into the low-memory span: accept an
entry whose base equals the current LowMemory and advance LowMemory by
its length. LowMemory starts at zero, so the first accepted block at
address 0 still starts the sequence; non-adjacent above-4G or SVSM-carved
blocks continue to be skipped (their base does not match LowMemory); and
the TDVF accepted+unaccepted pair, which is contiguous, is now grouped
correctly.

Co-authored-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stanislaw Grams <stanislaw.grams@intel.com>
This commit is contained in:
Stanislaw Grams 2026-07-03 13:31:38 +02:00 • committed by mergify[bot]
parent 3716793e66
commit c0c8ca0ea0
1 changed files with 7 additions and 3 deletions
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10
OvmfPkg/Library/PlatformInitLib/MemDetect.c
View file

@ -164,6 +164,10 @@ PlatformGetFirstNonAddressCB (
there are multiple memory blocks below 4G though, because SVSM caves out a
chunk of memory for itself. Only the first of these blocks is considered
low memory.
Multiple blocks without gap inbetween are grouped together.
This is required for TDX which has two low memory descriptors
(accepted and unaccepted).
**/
STATIC
VOID
@ -176,12 +180,12 @@ PlatformGetLowMemoryCB (
return;
}
if (E820Entry->BaseAddr != 0) {
if (E820Entry->BaseAddr != PlatformInfoHob->LowMemory) {
return;
}
DEBUG ((DEBUG_INFO, "%a: LowMemory=0x%Lx\n", __func__, E820Entry->Length));
PlatformInfoHob->LowMemory = (UINT32)E820Entry->Length;
PlatformInfoHob->LowMemory += (UINT32)E820Entry->Length;
DEBUG ((DEBUG_INFO, "%a: LowMemory=0x%Lx\n", __func__, PlatformInfoHob->LowMemory));
}
/**