Commit 4a5733771e6f ("libxl: put RSDP for PVH guest near 4GB") breaks
pre-4.17 Linux guests since they do not use start_info's rsdp_paddr
pointer and instread scan BIOS memory for RSDP signature.

Introduce XENFEAT_rsdp_unrestricted feature flag that indicates whether
the guest can handle RSDP at locations pointed to by rsdp_paddr.

Since only Linux PVH guests suffer from this problem (BSD has always
relied on rsdp_paddr) we check this flag just for those guests. If the
flag is not set we place RSDP in BIOS, as before.

Signed-off-by: Boris Ostrovsky <boris.ostrov...@oracle.com>
---
v2:
* Renamed flag to XENFEAT_linux_rsdp_unrestricted
* Clarified comment in features.h

I did not include tags because of the changes but just in case ---
here they are:

Acked-by: George Dunlap <george.dun...@citrix.com>
Reviewed-by: Juergen Gross <jgr...@suse.com>
Release-acked-by: Juergen Gross <jgr...@suse.com>


 tools/libxl/libxl_x86_acpi.c  | 14 ++++++++++++--
 xen/include/public/features.h |  9 +++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl_x86_acpi.c b/tools/libxl/libxl_x86_acpi.c
index fe87418bc1..143ce66644 100644
--- a/tools/libxl/libxl_x86_acpi.c
+++ b/tools/libxl/libxl_x86_acpi.c
@@ -218,8 +218,18 @@ int libxl__dom_load_acpi(libxl__gc *gc,
 
     dom->acpi_modules[0].data = (void *)config.rsdp;
     dom->acpi_modules[0].length = 64;
-    dom->acpi_modules[0].guest_addr_out = ACPI_INFO_PHYSICAL_ADDRESS +
-        (1 + acpi_pages_num) * libxl_ctxt.page_size;
+    /*
+     * Some Linux versions cannot properly process hvm_start_info.rsdp_paddr
+     * and so we need to put RSDP in location that can be discovered by ACPI's
+     * standard search method, in R-O BIOS memory (we chose last 64 bytes)
+     */
+    if (strcmp(dom->parms.guest_os, "linux") ||
+        elf_xen_feature_get(XENFEAT_linux_rsdp_unrestricted,
+                            dom->parms.f_supported))
+        dom->acpi_modules[0].guest_addr_out = ACPI_INFO_PHYSICAL_ADDRESS +
+            (1 + acpi_pages_num) * libxl_ctxt.page_size;
+    else
+        dom->acpi_modules[0].guest_addr_out = 0x100000 - 64;
 
     dom->acpi_modules[1].data = (void *)config.infop;
     dom->acpi_modules[1].length = 4096;
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index 1a989b8bf9..443e6eca53 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -105,6 +105,15 @@
 /* arm: Hypervisor supports ARM SMC calling convention. */
 #define XENFEAT_ARM_SMCCC_supported       14
 
+/*
+ * x86/PVH: If set, ACPI RSDP can be placed at any address. Otherwise RSDP
+ * must be located in lower 1MB, as required by ACPI Specification for IA-PC
+ * systems.
+ * This feature flag is only consulted if XEN_ELFNOTE_GUEST_OS contains
+ * the "linux" string.
+ */
+#define XENFEAT_linux_rsdp_unrestricted   15
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
-- 
2.14.3


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