[PATCH v4 2/2] Support ESRT in Xen dom0

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

fwupd requires access to the EFI System Resource Table (ESRT) to
discover which firmware can be updated by the OS.  Currently, Linux does
not expose the ESRT when running as a Xen dom0.  Therefore, it is not
possible to use fwupd in a Xen dom0, which is a serious problem for e.g.
Qubes OS.

Before Xen 4.17, this was not fixable due to hypervisor limitations.
The UEFI specification requires the ESRT to be in EfiBootServicesData
memory, which Xen will use for whatever purposes it likes.  Therefore,
Linux cannot safely access the ESRT, as Xen may have overwritten it.

Starting with Xen 4.17, Xen checks if the ESRT is in EfiBootServicesData
or EfiRuntimeServicesData memory.  If the ESRT is in EfiBootServicesData
memory, Xen replaces the ESRT with a copy in memory that it has
reserved.  Such memory is currently of type EFI_RUNTIME_SERVICES_DATA,
but in the future it will be of type EFI_ACPI_RECLAIM_MEMORY.  This
ensures that the ESRT can safely be accessed by the OS.

When running as a Xen dom0, use the new
xen_config_table_memory_region_max() function to determine if Xen has
reserved the ESRT and, if so, find the end of the memory region
containing it.  This allows programs such as fwupd which require the
ESRT to run under Xen, and so makes fwupd support in Qubes OS possible.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/firmware/efi/esrt.c | 43 ++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 13 deletions(-)

diff --git a/drivers/firmware/efi/esrt.c b/drivers/firmware/efi/esrt.c
index 
2a2f52b017e736dd995c69e8aeb5fbd7761732e5..a0642bc161b4b1f94f818b8c9f46511fe2424bb2
 100644
--- a/drivers/firmware/efi/esrt.c
+++ b/drivers/firmware/efi/esrt.c
@@ -243,27 +243,44 @@ void __init efi_esrt_init(void)
        void *va;
        struct efi_system_resource_table tmpesrt;
        size_t size, max, entry_size, entries_size;
-       efi_memory_desc_t md;
-       int rc;
        phys_addr_t end;
-
-       if (!efi_enabled(EFI_MEMMAP))
-               return;
+       u32 type;
 
        pr_debug("esrt-init: loading.\n");
        if (!esrt_table_exists())
                return;
 
-       rc = efi_mem_desc_lookup(efi.esrt, &md);
-       if (rc < 0 ||
-           (!(md.attribute & EFI_MEMORY_RUNTIME) &&
-            md.type != EFI_BOOT_SERVICES_DATA &&
-            md.type != EFI_RUNTIME_SERVICES_DATA)) {
-               pr_warn("ESRT header is not in the memory map.\n");
+       if (efi_enabled(EFI_MEMMAP)) {
+               efi_memory_desc_t md;
+
+               if (efi_mem_desc_lookup(efi.esrt, &md) < 0 ||
+                   (!(md.attribute & EFI_MEMORY_RUNTIME) &&
+                    md.type != EFI_BOOT_SERVICES_DATA &&
+                    md.type != EFI_RUNTIME_SERVICES_DATA)) {
+                       pr_warn("ESRT header is not in the memory map.\n");
+                       return;
+               }
+
+               type = md.type;
+               max = efi_mem_desc_end(&md);
+#ifdef CONFIG_XEN_EFI
+       } else if (efi_enabled(EFI_PARAVIRT)) {
+               max = xen_config_table_memory_region_max(efi.esrt);
+               /*
+                * This might be wrong, but it doesn't matter.
+                * xen_config_table_memory_region_max() checks the type
+                * of the memory region, and if it returns 0, the code
+                * below will fail without looking at the type.  Choose
+                * a value that will not cause * subsequent code to try
+                * to reserve the memory containing the ESRT, as either
+                * Xen or the firmware has done so already.
+                */
+               type = EFI_RUNTIME_SERVICES_DATA;
+#endif
+       } else {
                return;
        }
 
-       max = efi_mem_desc_end(&md);
        if (max < efi.esrt) {
                pr_err("EFI memory descriptor is invalid. (esrt: %p max: %p)\n",
                       (void *)efi.esrt, (void *)max);
@@ -333,7 +350,7 @@ void __init efi_esrt_init(void)
 
        end = esrt_data + size;
        pr_info("Reserving ESRT space from %pa to %pa.\n", &esrt_data, &end);
-       if (md.type == EFI_BOOT_SERVICES_DATA)
+       if (type == EFI_BOOT_SERVICES_DATA)
                efi_mem_reserve(esrt_data, esrt_data_size);
 
        pr_debug("esrt-init: loaded.\n");
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab