Re: vmm timer for linux guests

[Pratik Vyas]

* Renato Aguiar <ren...@renatoaguiar.net> [2020-05-21 12:55:45 -0700]:
Hi Sivaram,

I'm the author of the e-mail thread that you mentioned. After feedback 
I got from OpenBSD community, I created a patch for Linux to enable 
kvm-clock when booting on VMM. It managed to keep clock in sync, but I 
experienced random crashes in vmd after some time running the VM.

Multiple fixes have been merged to vmm/vmd since then, so I'm planning 
to give it another try with OpenBSD 6.7.

If you are interested, I can share the patch with you.

Regards,

--
Renato Aguiar


Hi Sivaram and Renato,

I have been looking at this issue this week with Dave.  (funnily Dave
and I independently also decided to write a linux patch to attach
pvclock and debugging these crashes).

If you have your linux vm with kvm-clock around, can you try this rather
crude patch and see if it still crashes?  This might also fix the '100%
cpu when using alpine via console' problem.

For ref, this is my linux side patch to attach kvm-clock http://ix.io/2lzK

--
Pratik

diff --git a/usr.sbin/vmd/ns8250.c b/usr.sbin/vmd/ns8250.c
index 497e6fad550..33f1a371c16 100644
--- a/usr.sbin/vmd/ns8250.c
+++ b/usr.sbin/vmd/ns8250.c
@@ -36,6 +36,8 @@
 
 extern char *__progname;
 struct ns8250_dev com1_dev;
+struct event read_delay_ev;
+struct timeval read_delay_tv;
 
 static void com_rcv_event(int, short, void *);
 static void com_rcv(struct ns8250_dev *, uint32_t, uint32_t);
@@ -61,6 +63,11 @@ ratelimit(int fd, short type, void *arg)
        vcpu_deassert_pic_irq(com1_dev.vmid, 0, com1_dev.irq);
 }
 
+static void
+arm_read(int fd, short type, void *arg) {
+       event_add(&com1_dev.event, NULL);
+}
+
 void
 ns8250_init(int fd, uint32_t vmid)
 {
@@ -96,7 +103,7 @@ ns8250_init(int fd, uint32_t vmid)
         */
        com1_dev.pause_ct = (com1_dev.baudrate / 8) / 1000 * 10;
 
-	event_set(&com1_dev.event, com1_dev.fd, EV_READ | EV_PERSIST,
+       event_set(&com1_dev.event, com1_dev.fd, EV_READ,
            com_rcv_event, (void *)(intptr_t)vmid);
 
 	/*
@@ -112,6 +119,9 @@ ns8250_init(int fd, uint32_t vmid)
        timerclear(&com1_dev.rate_tv);
        com1_dev.rate_tv.tv_usec = 10000;
        evtimer_set(&com1_dev.rate, ratelimit, NULL);
+       read_delay_tv.tv_usec = 10000;
+       evtimer_set(&read_delay_ev, arm_read,
+                       (void *)(intptr_t)vmid);
 }
 
 static void
@@ -131,6 +141,7 @@ com_rcv_event(int fd, short kind, void *arg)
         */
        if (com1_dev.rcv_pending) {
                mutex_unlock(&com1_dev.mutex);
+               evtimer_add(&read_delay_ev, &read_delay_tv);
                return;
        }
 
@@ -146,6 +157,7 @@ com_rcv_event(int fd, short kind, void *arg)
                        vcpu_deassert_pic_irq((uintptr_t)arg, 0, com1_dev.irq);
                }
        }
+       event_add(&com1_dev.event, NULL);
 
 	mutex_unlock(&com1_dev.mutex);
 }