AMD/IOMMU: size command buffer dynamically

With the present synchronous model, we need two slots for every
operation (the operation itself and a wait command).  There can be one
such pair of commands pending per CPU. To ensure that under all normal
circumstances a slot is always available when one is requested, size the
command ring according to the number of present CPUs.

This is part of XSA-373 / CVE-2021-28692.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
(cherry picked from commit df242851dd)

xen/drivers/passthrough/amd/iommu_cmd.c

@@ -35,8 +35,8 @@ static int queue_iommu_command(struct amd_iommu *iommu, u32 cmd[])
     if ( head != tail )
     {
         memcpy(iommu->cmd_buffer.buffer +
-               (iommu->cmd_buffer.tail * IOMMU_CMD_BUFFER_ENTRY_SIZE),
-               cmd, IOMMU_CMD_BUFFER_ENTRY_SIZE);
+               (iommu->cmd_buffer.tail * sizeof(cmd_entry_t)),
+               cmd, sizeof(cmd_entry_t));
 
         iommu->cmd_buffer.tail = tail;
         return 1;

xen/drivers/passthrough/amd/iommu_init.c

@@ -125,7 +125,7 @@ static void register_iommu_cmd_buffer_in_mmio_space(struct amd_iommu *iommu)
     writel(entry, iommu->mmio_base + IOMMU_CMD_BUFFER_BASE_LOW_OFFSET);
 
     power_of2_entries = get_order_from_bytes(iommu->cmd_buffer.alloc_size) +
-        IOMMU_CMD_BUFFER_POWER_OF2_ENTRIES_PER_PAGE;
+        PAGE_SHIFT - IOMMU_CMD_BUFFER_ENTRY_ORDER;
 
     entry = 0;
     iommu_set_addr_hi_to_reg(&entry, addr_hi);
@@ -1050,9 +1050,31 @@ static void *__init allocate_ring_buffer(struct ring_buffer *ring_buf,
 static void * __init allocate_cmd_buffer(struct amd_iommu *iommu)
 {
     /* allocate 'command buffer' in power of 2 increments of 4K */
+    static unsigned int __read_mostly nr_ents;
+
+    if ( !nr_ents )
+    {
+        unsigned int order;
+
+        /*
+         * With the present synchronous model, we need two slots for every
+         * operation (the operation itself and a wait command).  There can be
+         * one such pair of requests pending per CPU.  One extra entry is
+         * needed as the ring is considered full when there's only one entry
+         * left.
+         */
+        BUILD_BUG_ON(CONFIG_NR_CPUS * 2 >= IOMMU_CMD_BUFFER_MAX_ENTRIES);
+        order = get_order_from_bytes((num_present_cpus() * 2 + 1) <<
+                                     IOMMU_CMD_BUFFER_ENTRY_ORDER);
+        nr_ents = 1u << (order + PAGE_SHIFT - IOMMU_CMD_BUFFER_ENTRY_ORDER);
+
+        AMD_IOMMU_DEBUG("using %u-entry cmd ring(s)\n", nr_ents);
+    }
+
+    BUILD_BUG_ON(sizeof(cmd_entry_t) != (1u << IOMMU_CMD_BUFFER_ENTRY_ORDER));
+
     return allocate_ring_buffer(&iommu->cmd_buffer, sizeof(cmd_entry_t),
-                                IOMMU_CMD_BUFFER_DEFAULT_ENTRIES,
-                                "Command Buffer", false);
+                                nr_ents, "Command Buffer", false);
 }
 
 static void * __init allocate_event_log(struct amd_iommu *iommu)

xen/include/asm-x86/hvm/svm/amd-iommu-defs.h

@@ -20,9 +20,6 @@
 #ifndef _ASM_X86_64_AMD_IOMMU_DEFS_H
 #define _ASM_X86_64_AMD_IOMMU_DEFS_H
 
-/* IOMMU Command Buffer entries: in power of 2 increments, minimum of 256 */
-#define IOMMU_CMD_BUFFER_DEFAULT_ENTRIES	512
-
 /* IOMMU Event Log entries: in power of 2 increments, minimum of 256 */
 #define IOMMU_EVENT_LOG_DEFAULT_ENTRIES     512
 
@@ -168,8 +165,8 @@ struct amd_iommu_dte {
 #define IOMMU_CMD_BUFFER_LENGTH_MASK		0x0F000000
 #define IOMMU_CMD_BUFFER_LENGTH_SHIFT		24
 
-#define IOMMU_CMD_BUFFER_ENTRY_SIZE			16
-#define IOMMU_CMD_BUFFER_POWER_OF2_ENTRIES_PER_PAGE	8
+#define IOMMU_CMD_BUFFER_ENTRY_ORDER            4
+#define IOMMU_CMD_BUFFER_MAX_ENTRIES            (1u << 15)
 
 #define IOMMU_CMD_OPCODE_MASK			0xF0000000
 #define IOMMU_CMD_OPCODE_SHIFT			28