[PATCH v6 06/15] emul/ns16x50: implement IER/IIR registers

[dmukhin@xxxxxxx]

From: Denis Mukhin <dmukhin@xxxxxxxx> 

Add interrupt enable register emulation (IER) and interrupt identity reason
(IIR) register emulation to the I/O port handler.

Also add routines for asserting/deasserting the virtual ns16x50 interrupt
line as a dependent on IIR code.

Poke ns16x50_irq_check() on every I/O register access because the emulator
does not have clock emulation anyway (e.g. for baud rate emulation).

Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
---
Changes since v5:
- stubbed out ns16x50_iir_check_xxx()
- dropped UART_IIR_THR handling (moved to THR register patch)
- Link to v5: 
https://lore.kernel.org/xen-devel/20250828235409.2835815-6-dmukhin@xxxxxxxx/
---
 xen/common/emul/vuart/ns16x50.c | 135 ++++++++++++++++++++++++++++++++
 1 file changed, 135 insertions(+)

diff --git a/xen/common/emul/vuart/ns16x50.c b/xen/common/emul/vuart/ns16x50.c
index 51ec85e57627..a7429c84c36e 100644
--- a/xen/common/emul/vuart/ns16x50.c
+++ b/xen/common/emul/vuart/ns16x50.c
@@ -91,6 +91,121 @@ static uint8_t ns16x50_dlab_get(const struct vuart_ns16x50 
*vdev)
     return 0;
 }
 
+static bool cf_check ns16x50_iir_check_lsi(const struct vuart_ns16x50 *vdev)
+{
+    return false;
+}
+
+static bool cf_check ns16x50_iir_check_rda(const struct vuart_ns16x50 *vdev)
+{
+    return false;
+}
+
+static bool cf_check ns16x50_iir_check_thr(const struct vuart_ns16x50 *vdev)
+{
+    return false;
+}
+
+static bool cf_check ns16x50_iir_check_msi(const struct vuart_ns16x50 *vdev)
+{
+    return false;
+}
+
+/*
+ * Get the interrupt identity reason.
+ *
+ * IIR is re-calculated once called, because ns16x50 always reports high
+ * priority events first.
+ * regs[NS16X50_REGS_NUM + UART_IIR] is used to store THR reason only.
+ */
+static uint8_t ns16x50_iir_get(const struct vuart_ns16x50 *vdev)
+{
+    /*
+     * Interrupt identity reasons by priority.
+     * NB: high priority are at lower indexes below.
+     */
+    static const struct {
+        bool (*check)(const struct vuart_ns16x50 *vdev);
+        uint8_t ier;
+        uint8_t iir;
+    } iir_by_prio[] = {
+        [0] = { ns16x50_iir_check_lsi, UART_IER_ELSI,   UART_IIR_LSI },
+        [1] = { ns16x50_iir_check_rda, UART_IER_ERDAI,  UART_IIR_RDA },
+        [2] = { ns16x50_iir_check_thr, UART_IER_ETHREI, UART_IIR_THR },
+        [3] = { ns16x50_iir_check_msi, UART_IER_EMSI,   UART_IIR_MSI },
+    };
+    const uint8_t *regs = vdev->regs;
+    uint8_t iir = 0;
+    unsigned int i;
+
+    /*
+     * NB: every interaction w/ ns16x50 registers (except DLAB=1) goes
+     * through that call.
+     */
+    ASSERT(spin_is_locked(&vdev->lock));
+
+    for ( i = 0; i < ARRAY_SIZE(iir_by_prio); i++ )
+    {
+        if ( (regs[UART_IER] & iir_by_prio[i].ier) &&
+             iir_by_prio[i].check(vdev) )
+            break;
+
+    }
+    if ( i == ARRAY_SIZE(iir_by_prio) )
+        iir |= UART_IIR_NOINT;
+    else
+        iir |= iir_by_prio[i].iir;
+
+    if ( regs[UART_FCR] & UART_FCR_ENABLE )
+        iir |= UART_IIR_FE;
+
+    return iir;
+}
+
+static void ns16x50_irq_assert(const struct vuart_ns16x50 *vdev)
+{
+    struct domain *d = vdev->owner;
+    const struct vuart_info *info = vdev->info;
+    int vector;
+
+    if ( has_vpic(d) )
+        vector = hvm_isa_irq_assert(d, info->irq, vioapic_get_vector);
+    else
+        ASSERT_UNREACHABLE();
+
+    ns16x50_debug(vdev, "IRQ#%d vector %d assert\n", info->irq, vector);
+}
+
+static void ns16x50_irq_deassert(const struct vuart_ns16x50 *vdev)
+{
+    struct domain *d = vdev->owner;
+    const struct vuart_info *info = vdev->info;
+
+    if ( has_vpic(d) )
+        hvm_isa_irq_deassert(d, info->irq);
+    else
+        ASSERT_UNREACHABLE();
+
+    ns16x50_debug(vdev, "IRQ#%d deassert\n", info->irq);
+}
+
+/*
+ * Assert/deassert virtual ns16x50 interrupt line.
+ */
+static void ns16x50_irq_check(const struct vuart_ns16x50 *vdev)
+{
+    uint8_t iir = ns16x50_iir_get(vdev);
+    const struct vuart_info *info = vdev->info;
+
+    if ( iir & UART_IIR_NOINT )
+        ns16x50_irq_deassert(vdev);
+    else
+        ns16x50_irq_assert(vdev);
+
+    ns16x50_debug(vdev, "IRQ#%d IIR 0x%02x %s\n", info->irq, iir,
+                  (iir & UART_IIR_NOINT) ? "deassert" : "assert");
+}
+
 /*
  * Emulate 8-bit write access to ns16x50 register.
  */
@@ -107,6 +222,10 @@ static int ns16x50_io_write8(
     {
         switch ( reg )
         {
+        case UART_IER:
+            regs[UART_IER] = val & UART_IER_MASK;
+            break;
+
         /* NB: Firmware (e.g. OVMF) may rely on SCR presence. */
         case UART_SCR:
             regs[UART_SCR] = val;
@@ -116,6 +235,8 @@ static int ns16x50_io_write8(
             rc = -EINVAL;
             break;
         }
+
+        ns16x50_irq_check(vdev);
     }
 
     return rc;
@@ -183,6 +304,14 @@ static int ns16x50_io_read8(
     {
         switch ( reg )
         {
+        case UART_IER:
+            val = regs[UART_IER];
+            break;
+
+        case UART_IIR: /* RO */
+            val = ns16x50_iir_get(vdev);
+            break;
+
         case UART_SCR:
             val = regs[UART_SCR];
             break;
@@ -191,6 +320,8 @@ static int ns16x50_io_read8(
             rc = -EINVAL;
             break;
         }
+
+        ns16x50_irq_check(vdev);
     }
 
     *data = val;
@@ -344,6 +475,10 @@ static int ns16x50_init(void *arg)
 
     register_portio_handler(d, info->base_addr, info->size, ns16x50_io_handle);
 
+    spin_lock(&vdev->lock);
+    ns16x50_irq_check(vdev);
+    spin_unlock(&vdev->lock);
+
     return 0;
 }
 
-- 
2.51.0