x86/tdx: Make pages shared in ioremap()

In TDX guests, guest memory is protected from host access. If a guest
performs I/O, it needs to explicitly share the I/O memory with the host.

Make all ioremap()ed pages that are not backed by normal memory
(IORES_DESC_NONE or IORES_DESC_RESERVED) mapped as shared.

The permissions in PAGE_KERNEL_IO already work for "decrypted" memory
on AMD SEV/SME systems.  That means that they have no need to make a
pgprot_decrypted() call.

TDX guests, on the other hand, _need_ change to PAGE_KERNEL_IO for
"decrypted" mappings.  Add a pgprot_decrypted() for TDX.
Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20220405232939.73860-26-kirill.shutemov@linux.intel.com

arch/x86/mm/ioremap.c

@@ -242,10 +242,15 @@ __ioremap_caller(resource_size_t phys_addr, unsigned long size,
 	 * If the page being mapped is in memory and SEV is active then
 	 * make sure the memory encryption attribute is enabled in the
 	 * resulting mapping.
+	 * In TDX guests, memory is marked private by default. If encryption
+	 * is not requested (using encrypted), explicitly set decrypt
+	 * attribute in all IOREMAPPED memory.
 	 */
 	prot = PAGE_KERNEL_IO;
 	if ((io_desc.flags & IORES_MAP_ENCRYPTED) || encrypted)
 		prot = pgprot_encrypted(prot);
+	else
+		prot = pgprot_decrypted(prot);
 
 	switch (pcm) {
 	case _PAGE_CACHE_MODE_UC: