Re: [Qemu-devel] [RFC PATCH v2] qemu: x86: ignore ioapic polarity

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 28, 2014 at 01:57:19PM -0500, Gabriel L. Somlo wrote:
> Both QEMU and KVM have already accumulated a significant number of
> optimizations based on the hard-coded assumption that ioapic polarity
> will always use the ActiveHigh convention, where the logical and
> physical states of level-triggered irq lines always match (i.e.,
> active(asserted) == high == 1, inactive == low == 0). QEMU guests
> are expected to follow directions given via ACPI and configure the
> ioapic with polarity 0 (ActiveHigh). However, even when misbehaving
> guests (e.g. OS X <= 10.9) set the ioapic polarity to 1 (ActiveLow),
> QEMU will still use the ActiveHigh signaling convention when
> interfacing with the emulated ioapic.
> 
> This patch modifies the emulated ioapic to completely ignore polarity
> as set by the guest OS, enabling misbehaving guests to work alongside
> those which comply with the ActiveHigh polarity specified by QEMU's
> ACPI tables.
> 
> Signed-off-by: Gabriel L. Somlo <somlo@cmu.edu>


Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>


With this, we should be able to change ACPI to specify
active-low, which is closer to what real hardware does.
But as long as we don't I think there's no need for
compatibility with old machine types: we are not
breaking any behaviour that wasn't already broken.


> ---
> 
> > OK, this would "harmonize" TCG with KVM, in terms of acknowledging
> > the realities of hard-coded ActiveHigh behavior throughout the rest
> > of the code base.
> 
> Now with a new and improved commit blurb :)
> 
> Thanks,
>   Gabriel
> 
>  hw/intc/ioapic.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/hw/intc/ioapic.c b/hw/intc/ioapic.c
> index 652dd47..b527932 100644
> --- a/hw/intc/ioapic.c
> +++ b/hw/intc/ioapic.c
> @@ -93,9 +93,6 @@ static void ioapic_set_irq(void *opaque, int vector, int level)
>          uint32_t mask = 1 << vector;
>          uint64_t entry = s->ioredtbl[vector];
>  
> -        if (entry & (1 << IOAPIC_LVT_POLARITY_SHIFT)) {
> -            level = !level;
> -        }
>          if (((entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1) ==
>              IOAPIC_TRIGGER_LEVEL) {
>              /* level triggered */
> -- 
> 1.8.1.4