x86/ioremap: Add hypervisor callback for private MMIO mapping in coco VM

Current code always maps MMIO devices as shared (decrypted) in a
confidential computing VM. But Hyper-V guest VMs on AMD SEV-SNP with vTOM
use a paravisor running in VMPL0 to emulate some devices, such as the
IO-APIC and TPM. In such a case, the device must be accessed as private
(encrypted) because the paravisor emulates the device at an address below
vTOM, where all accesses are encrypted.

Add a new hypervisor callback to determine if an MMIO address should
be mapped private. The callback allows hypervisor-specific code to handle
any quirks, the use of a paravisor, etc. in determining whether a mapping
must be private. If the callback is not used by a hypervisor, default
to returning "false", which is consistent with normal coco VM behavior.

Use this callback as another special case to check for when doing
ioremap().  Just checking the starting address is sufficient as an
ioremap range must be all private or all shared.

Also make the callback in early boot IO-APIC mapping code that uses the
fixmap.

  [ bp: Touchups. ]

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/1678329614-3482-2-git-send-email-mikelley@microsoft.com

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 1f83b052bb74..146671de9ddc 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -66,6 +66,7 @@
 #include <asm/hw_irq.h>
 #include <asm/apic.h>
 #include <asm/pgtable.h>
+#include <asm/x86_init.h>
 
 #define	for_each_ioapic(idx)		\
 	for ((idx) = 0; (idx) < nr_ioapics; (idx)++)
@@ -2680,10 +2681,15 @@ static void io_apic_set_fixmap(enum fixed_addresses idx, phys_addr_t phys)
 	pgprot_t flags = FIXMAP_PAGE_NOCACHE;
 
 	/*
-	 * Ensure fixmaps for IOAPIC MMIO respect memory encryption pgprot
+	 * Ensure fixmaps for IO-APIC MMIO respect memory encryption pgprot
 	 * bits, just like normal ioremap():
 	 */
-	flags = pgprot_decrypted(flags);
+	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) {
+		if (x86_platform.hyper.is_private_mmio(phys))
+			flags = pgprot_encrypted(flags);
+		else
+			flags = pgprot_decrypted(flags);
+	}
 
 	__set_fixmap(idx, phys, flags);
 }