Re: [PATCH] xen: Remove event channel notification through Xen PCI platform device

[Boris Ostrovsky]

On 08/26/2016 05:55 PM, KarimAllah Ahmed wrote:
> Ever since commit 254d1a3f02eb ("xen/pv-on-hvm kexec: shutdown watches
> from old kernel") using the INTx interrupt from Xen PCI platform device for
> event channel notification would just lockup the guest during bootup.
> postcore_initcall now calls xs_reset_watches which will eventually try to read
> a value from XenStore and will get stuck on read_reply at XenBus forever since
> the platform driver is not probed yet and its INTx interrupt handler is not
> registered yet. That means that the guest can not be notified at this moment of
> any pending event channels and none of the per-event handlers will ever be
> invoked (including the XenStore one) and the reply will never be picked up by
> the kernel.
>
> The exact stack where things get stuck during xenbus_init:
>
> -xenbus_init
>  -xs_init
>   -xs_reset_watches
>    -xenbus_scanf
>     -xenbus_read
>      -xs_single
>       -xs_single
>        -xs_talkv
>
> Vector callbacks have always been the favourite event notification mechanism
> since their introduction in commit 38e20b07efd5 ("x86/xen: event channels
> delivery on HVM.") and the vector callback feature has always been advertised
> for quite some time by Xen that's why INTx was broken for several years now
> without impacting anyone.
>
> Luckily this also means that event channel notification through INTx is
> basically dead-code which can be safely removed without impacting anybody since
> it has been effectively disabled for more than 4 years with nobody complaining
> about it (at least as far as I'm aware of).
>
> This commit removes event channel notification through Xen PCI platform device.
>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-pci@xxxxxxxxxxxxxxx
> Cc: Anthony Liguori <aliguori@xxxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>