On Thu, Dec 05, 2024 at 08:41:53PM -0800, Denis Mukhin via B4 Relay wrote:
> From: Denis Mukhin <dmuk...@ford.com>
>
> PV Linux kernel uses HYPERVISOR_console_io hypercall for early console which
> ends up being handled by Xen's console driver's guest_console_write().
>
> guest_console_write() duplicates the code from __putstr(), elimitate code
> duplication.
It might be better to split the code that unifies
guest_console_write() and __putstr() as a non-functional change.
While the introduction of use_conring is likely a functional change.
> Signed-off-by: Denis Mukhin <dmuk...@ford.com>
> ---
> xen/drivers/char/console.c | 97
> +++++++++++++++++++++++++---------------------
> 1 file changed, 53 insertions(+), 44 deletions(-)
>
> diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> index
> ce3639a4cdcda00ea63e3bf119bc3b242cbfdf6a..115967d179998cba4a81578caba09db4e4aca7f7
> 100644
> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -63,6 +63,8 @@ static const char __initconst warning_sync_console[] =
> "However it can introduce SIGNIFICANT latencies and affect\n"
> "timekeeping. It is NOT recommended for production use!\n";
>
> +/* Flag: use conring for early console; switches to opt_console_to_ring */
> +static bool __read_mostly use_conring = true;
__ro_after_init instead of __read_mostly.
> /* console_to_ring: send guest (incl. dom 0) console data to console ring. */
> static bool __read_mostly opt_console_to_ring;
> boolean_param("console_to_ring", opt_console_to_ring);
> @@ -661,6 +663,16 @@ static void cf_check notify_dom0_con_ring(void *unused)
> static DECLARE_SOFTIRQ_TASKLET(notify_dom0_con_ring_tasklet,
> notify_dom0_con_ring, NULL);
>
> +static bool console_locks_busted;
> +
> +static void conring_write(const char *str, size_t len)
> +{
> + conring_puts(str, len);
> +
> + if ( !console_locks_busted )
> + tasklet_schedule(¬ify_dom0_con_ring_tasklet);
> +}
> +
> #ifdef CONFIG_X86
> static inline void xen_console_write_debug_port(const char *buf, size_t len)
> {
> @@ -669,8 +681,44 @@ static inline void xen_console_write_debug_port(const
> char *buf, size_t len)
> : "=&S" (tmp), "=&c" (tmp)
> : "0" (buf), "1" (len), "d" (XEN_HVM_DEBUGCONS_IOPORT) );
> }
> +
> +static void xen_console_write(const char *str, size_t len)
> +{
> + if ( xen_guest )
> + xen_hypercall_console_write(str, len);
> + else
> + xen_console_write_debug_port(str, len);
> +}
> +#else
> +static inline void xen_console_write(const char *str, size_t len)
> +{
opt_console_xen would only be set on x86 with the current command line
parsing done in console_init_preirq(), so you could add an
ASSERT_UNREACHABLE() here.
> +}
> #endif
>
> +/*
> + * Write characters to console.
> + *
> + * That will handle all possible scenarios working w/ console
> + * - serial console;
> + * - video output;
> + * - __HYPERVISOR_console_io hypercall (x86 only);
> + * - debug I/O port (x86 only);
> + * - forward to Xen event channel.
"Xen event channel" is not the correct term. I would use "PV
console". The event channel is just used to send the notification.
> + */
> +static void console_write(const char *str, size_t len)
> +{
> + ASSERT(rspin_is_locked(&console_lock));
> +
> + console_serial_puts(str, len);
> + video_puts(str, len);
> +
> + if ( opt_console_xen )
> + xen_console_write(str, len);
Are you sure this builds? opt_console_xen is only defined on x86, and
AFAICT console_write() is generic. AFAICT you need to keep the X86
preprocessor guards, or alternatively do something like:
#define opt_console_xen false
For non-x86 arches in xen/console.h
> +
> + if ( use_conring )
> + conring_write(str, len);
> +}
> +
> static long guest_console_write(XEN_GUEST_HANDLE_PARAM(char) buffer,
> unsigned int count)
> {
> @@ -691,28 +739,8 @@ static long
> guest_console_write(XEN_GUEST_HANDLE_PARAM(char) buffer,
>
> if ( is_hardware_domain(cd) )
> {
> - /* Use direct console output as it could be interactive */
> nrspin_lock_irq(&console_lock);
> -
> - console_serial_puts(kbuf, kcount);
> - video_puts(kbuf, kcount);
> -
> -#ifdef CONFIG_X86
> - if ( opt_console_xen )
> - {
> - if ( xen_guest )
> - xen_hypercall_console_write(kbuf, kcount);
> - else
> - xen_console_write_debug_port(kbuf, kcount);
> - }
> -#endif
> -
> - if ( opt_console_to_ring )
> - {
> - conring_puts(kbuf, kcount);
> - tasklet_schedule(¬ify_dom0_con_ring_tasklet);
> - }
> -
> + console_write(kbuf, kcount);
> nrspin_unlock_irq(&console_lock);
> }
> else
> @@ -813,31 +841,9 @@ long do_console_io(
> * *****************************************************
> */
>
> -static bool console_locks_busted;
> -
> static void __putstr(const char *str)
> {
> - size_t len = strlen(str);
> -
> - ASSERT(rspin_is_locked(&console_lock));
> -
> - console_serial_puts(str, len);
> - video_puts(str, len);
> -
> -#ifdef CONFIG_X86
> - if ( opt_console_xen )
> - {
> - if ( xen_guest )
> - xen_hypercall_console_write(str, len);
> - else
> - xen_console_write_debug_port(str, len);
> - }
> -#endif
> -
> - conring_puts(str, len);
> -
> - if ( !console_locks_busted )
> - tasklet_schedule(¬ify_dom0_con_ring_tasklet);
> + console_write(str, strlen(str));
> }
>
> static int printk_prefix_check(char *p, char **pp)
> @@ -1171,6 +1177,9 @@ void __init console_endboot(void)
>
> video_endboot();
>
> + use_conring = opt_console_to_ring;
> + smp_wmb();
Do you really need the barrier? If so it would need a comment
describing exactly why it's needed. I don't think it's possible for
the write to be reordered past the return of the function, which would
be enough to ensure correctness?
Thanks, Roger.
