[Patch v4 01/13] x86/ioapic: Gate decrypted mapping on cc_platform_has() attribute

[Michael Kelley <mikelley@microsoft.com>]

Current code always maps the IO-APIC as shared (decrypted) in a
confidential VM. But Hyper-V guest VMs on AMD SEV-SNP with vTOM
enabled use a paravisor running in VMPL0 to emulate the IO-APIC.
In such a case, the IO-APIC must be accessed as private (encrypted).

Fix this by gating the IO-APIC decrypted mapping on a new
cc_platform_has() attribute that a subsequent patch in the series
will set only for guests using vTOM.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
---
 arch/x86/kernel/apic/io_apic.c |  3 ++-
 include/linux/cc_platform.h    | 12 ++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index a868b76..2b70e2e 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2686,7 +2686,8 @@ static void io_apic_set_fixmap(enum fixed_addresses idx, phys_addr_t phys)
 	 * Ensure fixmaps for IOAPIC MMIO respect memory encryption pgprot
 	 * bits, just like normal ioremap():
 	 */
-	flags = pgprot_decrypted(flags);
+	if (!cc_platform_has(CC_ATTR_ACCESS_IOAPIC_ENCRYPTED))
+		flags = pgprot_decrypted(flags);
 
 	__set_fixmap(idx, phys, flags);
 }
diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
index cb0d6cd..7b63a7d 100644
--- a/include/linux/cc_platform.h
+++ b/include/linux/cc_platform.h
@@ -90,6 +90,18 @@ enum cc_attr {
 	 * Examples include TDX Guest.
 	 */
 	CC_ATTR_HOTPLUG_DISABLED,
+
+	/**
+	 * @CC_ATTR_ACCESS_IOAPIC_ENCRYPTED: Guest VM IO-APIC is encrypted
+	 *
+	 * The platform/OS is running as a guest/virtual machine with
+	 * an IO-APIC that is emulated by a paravisor running in the
+	 * guest VM context. As such, the IO-APIC is accessed in the
+	 * encrypted portion of the guest physical address space.
+	 *
+	 * Examples include Hyper-V SEV-SNP guests using vTOM.
+	 */
+	CC_ATTR_ACCESS_IOAPIC_ENCRYPTED,
 };
 
 #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
-- 
1.8.3.1