[PATCH v4 1/8] serial: fake IRQ-regs context in poll handlers

[Jan Beulich <jbeulich@xxxxxxxx>]

In preparation of dropping the register parameters from
serial_[rt]x_interrupt() and in turn from IRQ handler functions,
register state needs making available another way for the few key
handlers which need it. Fake IRQ-like state.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The use of guest_cpu_user_regs() in dbc_uart_poll() is inconsistent with
other console poll functions we have, and it's unclear whether that's
actually generally correct. It is only for this reason that it doesn't
need changing here.

Andrew suggested to move set_irq_regs() to BUGFRAME_run_fn handling;
it's not clear to me whether that would be (a) correct from an abstract
pov (that's exception, not interrupt context after all) and (b) really
beneficial.
---
v4: Drop xhci-dbc.c change as unnecessary with dump_registers() falling
    back to using guest_cpu_user_regs() as of patch 2, i.e. ahead of
    serial_rx_interrupt() having its 2nd parameter dropped.
v2: New.

--- a/xen/drivers/char/ehci-dbgp.c
+++ b/xen/drivers/char/ehci-dbgp.c
@@ -1253,6 +1253,7 @@ static void cf_check _ehci_dbgp_poll(str
     unsigned long flags;
     unsigned int timeout = MICROSECS(DBGP_CHECK_INTERVAL);
     bool empty = false;
+    struct cpu_user_regs *old_regs;
 
     if ( !dbgp->ehci_debug )
         return;
@@ -1268,12 +1269,17 @@ static void cf_check _ehci_dbgp_poll(str
         spin_unlock_irqrestore(&port->tx_lock, flags);
     }
 
+    /* Mimic interrupt context. */
+    old_regs = set_irq_regs(regs);
+
     if ( dbgp->in.chunk )
         serial_rx_interrupt(port, regs);
 
     if ( empty )
         serial_tx_interrupt(port, regs);
 
+    set_irq_regs(old_regs);
+
     if ( spin_trylock_irqsave(&port->tx_lock, flags) )
     {
         if ( dbgp->state == dbgp_idle && !dbgp->in.chunk &&
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -211,10 +211,14 @@ static void cf_check __ns16550_poll(stru
 {
     struct serial_port *port = this_cpu(poll_port);
     struct ns16550 *uart = port->uart;
+    struct cpu_user_regs *old_regs;
 
     if ( uart->intr_works )
         return; /* Interrupts work - no more polling */
 
+    /* Mimic interrupt context. */
+    old_regs = set_irq_regs(regs);
+
     while ( ns_read_reg(uart, UART_LSR) & UART_LSR_DR )
     {
         if ( ns16550_ioport_invalid(uart) )
@@ -227,6 +231,7 @@ static void cf_check __ns16550_poll(stru
         serial_tx_interrupt(port, regs);
 
 out:
+    set_irq_regs(old_regs);
     set_timer(&uart->timer, NOW() + MILLISECS(uart->timeout_ms));
 }