[PATCH v2] x86/EISA: Use memremap() to probe for the EISA BIOS signature

[Maciej W. Rozycki]

Area at the 0x0FFFD9 physical location in the PC memory space is regular 
memory, traditionally ROM BIOS and more recently a copy of BIOS code and 
data in RAM, write-protected.

Use memremap() then to get access to it rather than ioremap(), avoiding 
issues in virtualization scenarios and complementing changes such as 
commit f7750a795687 ("x86, mpparse, x86/acpi, x86/PCI, x86/dmi, SFI: Use 
memremap() for RAM mappings") or commit 5997efb96756 ("x86/boot: Use 
memremap() to map the MPF and MPC data").

Reported-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/r/20240822095122.736522-1-kirill.shutemov@xxxxxxxxxxxxxxx
Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
---
Changes from v1:

- Access the signature directly rather than via `readl', fixing a sparse 
  warning found by CI.
---
 arch/x86/kernel/eisa.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

linux-x86-eisa-bus-probe-memremap.diff
Index: linux-macro/arch/x86/kernel/eisa.c
===================================================================
--- linux-macro.orig/arch/x86/kernel/eisa.c
+++ linux-macro/arch/x86/kernel/eisa.c
@@ -11,15 +11,15 @@
 
 static __init int eisa_bus_probe(void)
 {
-	void __iomem *p;
+	unsigned int *p;
 
 	if ((xen_pv_domain() && !xen_initial_domain()) || cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 		return 0;
 
-	p = ioremap(0x0FFFD9, 4);
-	if (p && readl(p) == 'E' + ('I' << 8) + ('S' << 16) + ('A' << 24))
+	p = memremap(0x0FFFD9, 4, MEMREMAP_WB);
+	if (p && *p == 'E' + ('I' << 8) + ('S' << 16) + ('A' << 24))
 		EISA_bus = 1;
-	iounmap(p);
+	memunmap(p);
 	return 0;
 }
 subsys_initcall(eisa_bus_probe);