[La1itchauhan]

Description

In Check_1 of p004 (non-prefetchable) and p005 (prefetchable), the test writes KNOWN_DATA (0xABABABAB) to the start of the Root Port's memory window at base + mem_offset to verify that in-range memory access completes without an exception or data abort.

Although old_value was read prior to writing, it was never restored back to memory upon completing the check or on exception recovery. For devices where the endpoint BAR hosts the MSI-X table at offset 0 (where pal_pcie_mem_get_offset() defaults to 0x10), this leaves 0xABABABAB in entry 1's Message Lower Address.

Subsequent tests in the test suite that inspect live MSI-X tables (such as PCI_MSI_2 / test 897 and S_L3GI_02 / test 846) then fail to resolve the corrupted vector through the ITS mappings and report false test failures.

Key Changes:

1. Normal Path Immediate Restore: Write old_value back to mem_base + mem_offset immediately after the in-window readback check in both p004.c and p005.c.
2. Exception Path Guard & Coherency: Guard exception recovery using volatile uint32_t restore_pending and volatile uint32_t old_value (read into temporary tmp to avoid discarded qualifier warnings). In exception_return:, after Root Port window registers are restored (TYPE1_NP_MEM / TYPE1_P_MEM), restore old_value at ori_mem_base + mem_offset. restore_pending is cleared prior to issuing the write to prevent recursive exception loops in case of access aborts.
3. Safe Context & Base Initialization: In p005.c, initialize ori_mem_base = mem_base; and refresh branch_stack_pointer = val_pe_save_exception_return_context(); before Check_1 (aligning with p004.c) so that any exception during Check_1 targets the correct original base on the current device. The duplicate ori_mem_base = mem_base; prior to Check_2 is retained for Check_2 logic.

Note on Platform Ports / PAL:

As discussed in #650, PAL implementers should also ensure that pal_pcie_mem_get_offset() returns an offset within BAR memory that is safe to access and overwrite without corrupting device configuration or capability structures (such as MSI-X tables). Restoring original BAR contents in the ACS test ensures test hygiene and robustness regardless of table layout.

Fixes #650

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Test Plan

• Compile-tested only: Verified compilation with aarch64-linux-gnu-gcc -Wall -Wextra -Werror -DTARGET_BAREMETAL across test_pool/pcie/p004.c and test_pool/pcie/p005.c with zero warnings and zero errors.
• Verified disassembly under both -O0 and -O2 to confirm old_value and restore_pending stack-slot reloads at exception_return:.
• Verified git diff --check passes cleanly with no whitespace or style issues.
• (Compile-tested only; baremetal platform execution with MSI-X table at offset 0 not available in local dev environment).