arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC

The only user we have of Normal Write-Through memory is in the ACPI code
when mapping memory regions advertised as EFI_MEMORY_WT. Since most (all?)
CPUs treat write-through as non-cacheable under the hood, don't bother
with the extra memory type here and just treat EFI_MEMORY_WT the same way
as EFI_MEMORY_WC by mapping it to the Normal-NC memory type instead and
emitting a warning if we have failed to find an alternative EFI memory
type.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210527110319.22157-3-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
index cada0b8..f385172 100644 (file)
--- a/arch/arm64/kernel/acpi.c
+++ b/arch/arm64/kernel/acpi.c
@@ -239,6 +239,18 @@ done:
        }
 }
 
+static pgprot_t __acpi_get_writethrough_mem_attribute(void)
+{
+       /*
+        * Although UEFI specifies the use of Normal Write-through for
+        * EFI_MEMORY_WT, it is seldom used in practice and not implemented
+        * by most (all?) CPUs. Rather than allocate a MAIR just for this
+        * purpose, emit a warning and use Normal Non-cacheable instead.
+        */
+       pr_warn_once("No MAIR allocation for EFI_MEMORY_WT; treating as Normal Non-cacheable\n");
+       return __pgprot(PROT_NORMAL_NC);
+}
+
 pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
 {
        /*
@@ -246,7 +258,7 @@ pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
         * types" of UEFI 2.5 section 2.3.6.1, each EFI memory type is
         * mapped to a corresponding MAIR attribute encoding.
         * The EFI memory attribute advises all possible capabilities
-        * of a memory region. We use the most efficient capability.
+        * of a memory region.
         */
 
        u64 attr;
@@ -254,10 +266,10 @@ pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
        attr = efi_mem_attributes(addr);
        if (attr & EFI_MEMORY_WB)
                return PAGE_KERNEL;
-       if (attr & EFI_MEMORY_WT)
-               return __pgprot(PROT_NORMAL_WT);
        if (attr & EFI_MEMORY_WC)
                return __pgprot(PROT_NORMAL_NC);
+       if (attr & EFI_MEMORY_WT)
+               return __acpi_get_writethrough_mem_attribute();
        return __pgprot(PROT_DEVICE_nGnRnE);
 }
 
@@ -340,10 +352,10 @@ void __iomem *acpi_os_ioremap(acpi_physical_address phys, acpi_size size)
                default:
                        if (region->attribute & EFI_MEMORY_WB)
                                prot = PAGE_KERNEL;
-                       else if (region->attribute & EFI_MEMORY_WT)
-                               prot = __pgprot(PROT_NORMAL_WT);
                        else if (region->attribute & EFI_MEMORY_WC)
                                prot = __pgprot(PROT_NORMAL_NC);
+                       else if (region->attribute & EFI_MEMORY_WT)
+                               prot = __acpi_get_writethrough_mem_attribute();
                }
        }
        return __ioremap(phys, size, prot);