efi: xen: Implement memory descriptor lookup based on hypercall

Xen on x86 boots dom0 in EFI mode but without providing a memory map.
This means that some consistency checks we would like to perform on
configuration tables or other data structures in memory are not
currently possible.  Xen does, however, expose EFI memory descriptor
info via a Xen hypercall, so let's wire that up instead.  It turns out
that the returned information is not identical to what Linux's
efi_mem_desc_lookup would return: the address returned is the address
passed to the hypercall, and the size returned is the number of bytes
remaining in the configuration table.  However, none of the callers of
efi_mem_desc_lookup() currently care about this.  In the future, Xen may
gain a hypercall that returns the actual start address, which can be
used instead.
Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/efi.c

@@ -478,7 +478,7 @@ void __init efi_find_mirror(void)
  * and if so, populate the supplied memory descriptor with the appropriate
  * data.
  */
-int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
+int __efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
 {
 	efi_memory_desc_t *md;
 
@@ -512,6 +512,9 @@ int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
 	return -ENOENT;
 }
 
+extern int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
+	__weak __alias(__efi_mem_desc_lookup);
+
 /*
  * Calculate the highest address of an efi memory descriptor.
  */

drivers/xen/efi.c

@@ -26,6 +26,7 @@
 
 #include <xen/interface/xen.h>
 #include <xen/interface/platform.h>
+#include <xen/page.h>
 #include <xen/xen.h>
 #include <xen/xen-ops.h>
 
@@ -292,3 +293,38 @@ void __init xen_efi_runtime_setup(void)
 	efi.get_next_high_mono_count	= xen_efi_get_next_high_mono_count;
 	efi.reset_system		= xen_efi_reset_system;
 }
+
+int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
+{
+	static_assert(XEN_PAGE_SHIFT == EFI_PAGE_SHIFT,
+	              "Mismatch between EFI_PAGE_SHIFT and XEN_PAGE_SHIFT");
+	struct xen_platform_op op;
+	union xenpf_efi_info *info = &op.u.firmware_info.u.efi_info;
+	int rc;
+
+	if (!efi_enabled(EFI_PARAVIRT) || efi_enabled(EFI_MEMMAP))
+		return __efi_mem_desc_lookup(phys_addr, out_md);
+	phys_addr &= ~(u64)(EFI_PAGE_SIZE - 1);
+	op = (struct xen_platform_op) {
+		.cmd = XENPF_firmware_info,
+		.u.firmware_info = {
+			.type = XEN_FW_EFI_INFO,
+			.index = XEN_FW_EFI_MEM_INFO,
+			.u.efi_info.mem.addr = phys_addr,
+			.u.efi_info.mem.size = U64_MAX - phys_addr,
+		},
+	};
+
+	rc = HYPERVISOR_platform_op(&op);
+	if (rc) {
+		pr_warn("Failed to lookup header 0x%llx in Xen memory map: error %d\n",
+		        phys_addr, rc);
+	}
+
+	out_md->phys_addr	= info->mem.addr;
+	out_md->num_pages	= info->mem.size >> EFI_PAGE_SHIFT;
+	out_md->type    	= info->mem.type;
+	out_md->attribute	= info->mem.attr;
+
+	return 0;
+}

include/linux/efi.h

@@ -731,6 +731,7 @@ extern u64 efi_mem_attribute (unsigned long phys_addr, unsigned long size);
 extern int __init efi_uart_console_only (void);
 extern u64 efi_mem_desc_end(efi_memory_desc_t *md);
 extern int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md);
+extern int __efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md);
 extern void efi_mem_reserve(phys_addr_t addr, u64 size);
 extern int efi_mem_reserve_persistent(phys_addr_t addr, u64 size);
 extern void efi_initialize_iomem_resources(struct resource *code_resource,