Hi Stefano, On 22/06/17 23:53, Stefano Stabellini wrote:
On Thu, 22 Jun 2017, Bhupinder Thakur wrote:+static void vpl011_write_data(struct domain *d, uint8_t data) +{ + unsigned long flags; + struct vpl011 *vpl011 = &d->arch.vpl011; + struct xencons_interface *intf = vpl011->ring_buf; + XENCONS_RING_IDX out_cons, out_prod; + + VPL011_LOCK(d, flags); + + out_cons = intf->out_cons; + out_prod = intf->out_prod; + + smp_rmb();This should be smp_mb();
To speed up discussion, it would have been nice to give a bit more details why you think smp_rmb() is not enough.
In this case, I think smp_rmb() is fine because all the write we care depends on out_cons and out_prod. So the processor cannot re-order it.
+ /* + * It is expected that the ring is not full when this function is called + * as the guest is expected to write to the data register only when the + * TXFF flag is not set. + * In case the guest does write even when the TXFF flag is set then the + * data will be silently dropped. + */ + if ( xencons_queued(out_prod, out_cons, sizeof(intf->out)) != + sizeof (intf->out) ) + { + intf->out[xencons_mask(out_prod, sizeof(intf->out))] = data; + out_prod += 1; + smp_wmb(); + intf->out_prod = out_prod; + } + else + gprintk(XENLOG_ERR, "vpl011: Unexpected OUT ring buffer full\n"); + + if ( xencons_queued(out_prod, out_cons, sizeof(intf->out)) == + sizeof (intf->out) ) + { + vpl011->uartfr |= TXFF; + vpl011->uartris &= ~TXI; + } + + vpl011->uartfr |= BUSY; + + vpl011->uartfr &= ~TXFE; + + vpl011_update_interrupt_status(d); + + VPL011_UNLOCK(d, flags); + + /* + * Send an event to console backend to indicate that there is + * data in the OUT ring buffer. + */ + notify_via_xen_event_channel(d, vpl011->evtchn); +}
Cheers, -- Julien Grall _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
