iommu/amd: Document which IRTE fields amd_iommu_update_ga() can modify

Add a comment to amd_iommu_update_ga() to document what fields it can
safely modify without issuing an invalidation of the IRTE, and to explain
its role in keeping GA IRTEs up-to-date.

Per page 93 of the IOMMU spec dated Feb 2025:

  When virtual interrupts are enabled by setting MMIO Offset 0018h[GAEn] and
  IRTE[GuestMode=1], IRTE[IsRun], IRTE[Destination], and if present IRTE[GATag],
  are not cached by the IOMMU. Modifications to these fields do not require an
  invalidation of the Interrupt Remapping Table.

Link: https://lore.kernel.org/all/9b7ceea3-8c47-4383-ad9c-1a9bbdc9044a@oracle.com
Cc: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20250611224604.313496-41-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 8366d32252cdbb0c560ad1c2bea2c1b27b1762da..0a0d7348319531f10f53e0e09dfae671e6c67ab1 100644 (file)
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -3986,6 +3986,18 @@ int amd_iommu_create_irq_domain(struct amd_iommu *iommu)
        return 0;
 }
 
+/*
+ * Update the pCPU information for an IRTE that is configured to post IRQs to
+ * a vCPU, without issuing an IOMMU invalidation for the IRTE.
+ *
+ * This API is intended to be used when a vCPU is scheduled in/out (or stops
+ * running for any reason), to do a fast update of IsRun and (conditionally)
+ * Destination.
+ *
+ * Per the IOMMU spec, the Destination, IsRun, and GATag fields are not cached
+ * and thus don't require an invalidation to ensure the IOMMU consumes fresh
+ * information.
+ */
 int amd_iommu_update_ga(int cpu, bool is_run, void *data)
 {
        struct amd_ir_data *ir_data = (struct amd_ir_data *)data;