[PATCH 1/9] AMD/IOMMU: redo awaiting of command completion

From: Jan Beulich <jbeulich@suse.com>
To: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Paul Durrant <paul@xen.org>, Andrew Cooper <andrew.cooper3@citrix.com>
Subject: [PATCH 1/9] AMD/IOMMU: redo awaiting of command completion
Date: Wed, 9 Jun 2021 11:26:48 +0200	[thread overview]
Message-ID: <1bb49cce-537e-9fba-80bb-82bf502728d0@suse.com> (raw)
In-Reply-To: <03285055-47ff-ab10-ae76-0553f28f136d@suse.com>

The present abuse of the completion interrupt does not only stand in the
way of, down the road, using it for its actual purpose, but also
requires holding the IOMMU lock while waiting for command completion,
limiting parallelism and keeping interrupts off for non-negligible
periods of time. Have the IOMMU do an ordinary memory write instead of
signaling an otherwise disabled interrupt (by just updating a status
register bit).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>

--- a/xen/drivers/passthrough/amd/iommu_cmd.c
+++ b/xen/drivers/passthrough/amd/iommu_cmd.c
@@ -20,6 +20,9 @@
 #include "iommu.h"
 #include "../ats.h"
 
+#define CMD_COMPLETION_INIT 0
+#define CMD_COMPLETION_DONE 1
+
 static void send_iommu_command(struct amd_iommu *iommu,
                                const uint32_t cmd[4])
 {
@@ -49,28 +52,31 @@ static void send_iommu_command(struct am
 static void flush_command_buffer(struct amd_iommu *iommu,
                                  unsigned int timeout_base)
 {
+    static DEFINE_PER_CPU(uint64_t, poll_slot);
+    uint64_t *this_poll_slot = &this_cpu(poll_slot);
+    paddr_t addr = virt_to_maddr(this_poll_slot);
     uint32_t cmd[4];
     s_time_t start, timeout;
     static unsigned int __read_mostly threshold = 1;
 
-    /* RW1C 'ComWaitInt' in status register */
-    writel(IOMMU_STATUS_COMP_WAIT_INT,
-           iommu->mmio_base + IOMMU_STATUS_MMIO_OFFSET);
-
-    /* send an empty COMPLETION_WAIT command to flush command buffer */
-    cmd[3] = cmd[2] = 0;
-    set_field_in_reg_u32(IOMMU_CMD_COMPLETION_WAIT, 0,
+    ACCESS_ONCE(*this_poll_slot) = CMD_COMPLETION_INIT;
+
+    /* send a COMPLETION_WAIT command to flush command buffer */
+    cmd[0] = addr;
+    set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, cmd[0],
+                         IOMMU_COMP_WAIT_S_FLAG_MASK,
+                         IOMMU_COMP_WAIT_S_FLAG_SHIFT, &cmd[0]);
+    cmd[1] = addr >> 32;
+    set_field_in_reg_u32(IOMMU_CMD_COMPLETION_WAIT, cmd[1],
                          IOMMU_CMD_OPCODE_MASK,
                          IOMMU_CMD_OPCODE_SHIFT, &cmd[1]);
-    set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, 0,
-                         IOMMU_COMP_WAIT_I_FLAG_MASK,
-                         IOMMU_COMP_WAIT_I_FLAG_SHIFT, &cmd[0]);
+    cmd[2] = CMD_COMPLETION_DONE;
+    cmd[3] = 0;
     send_iommu_command(iommu, cmd);
 
     start = NOW();
     timeout = start + (timeout_base ?: 100) * MILLISECS(threshold);
-    while ( !(readl(iommu->mmio_base + IOMMU_STATUS_MMIO_OFFSET) &
-              IOMMU_STATUS_COMP_WAIT_INT) )
+    while ( ACCESS_ONCE(*this_poll_slot) != CMD_COMPLETION_DONE )
     {
         if ( timeout && NOW() > timeout )
         {


