linux-kernel - Re: [PATCH] partially revert "xen: Remove event channel notification through Xen PCI platform device"

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Date:   Thu, 12 Jan 2017 13:33:59 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        karahmed@...zon.de, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] partially revert "xen: Remove event channel notification
 through Xen PCI platform device"

On Thu, 12 Jan 2017, Boris Ostrovsky wrote:
> On 01/11/2017 06:36 PM, Stefano Stabellini wrote:
> > The following commit:
> >
> > commit 72a9b186292d98494f222226cfd24a1621796209
> > Author: KarimAllah Ahmed <karahmed@...zon.de>
> > Date:   Fri Aug 26 23:55:36 2016 +0200
> >
> >     xen: Remove event channel notification through Xen PCI platform device
> >
> > broke Linux when booting as Dom0 on Xen in a nested Xen environment (Xen
> > installed inside a Xen VM). In this scenario, Linux is a PV guest, but
> > at the same time it uses the platform-pci driver to receive
> > notifications from L0 Xen. vector callbacks are not available because L1
> > Xen doesn't allow them.
> 
> (+Konrad who has been running nested)
> 
> >
> > Partially revert the offending commit, by restoring IRQ based
> > notifications for PV guests only. I restored only the code which is
> > strictly needed and replaced the xen_have_vector_callback checks within
> > it with xen_pv_domain() checks.
> >
> > Signed-off-by: Stefano Stabellini <sstabellini@...nel.org>
> >
> > ---
> > Alternatively, I could also restore the xen_have_vector_callback
> > checks. In general, it's best to have feature flag checks than umbrella
> > xen_pv/hvm_domain() checks.
> 
> I don't think it's worth doing given that we know that HVM we cant' run
> without this feature (we have
> BUG_ON(!xen_feature(XENFEAT_hvm_callback_vector)) in xen_hvm_guest_init()).

OK. I'll add an in-code comment.


> > ---
> >
> > diff --git a/drivers/xen/platform-pci.c b/drivers/xen/platform-pci.c
> > index b59c9455..1477f1d 100644
> > --- a/drivers/xen/platform-pci.c
> > +++ b/drivers/xen/platform-pci.c
> > @@ -42,6 +42,7 @@
> >  static unsigned long platform_mmio;
> >  static unsigned long platform_mmio_alloc;
> >  static unsigned long platform_mmiolen;
> > +static uint64_t callback_via;
> >  
> >  static unsigned long alloc_xen_mmio(unsigned long len)
> >  {
> > @@ -54,6 +55,51 @@ static unsigned long alloc_xen_mmio(unsigned long len)
> >  	return addr;
> >  }
> >  
> > +static uint64_t get_callback_via(struct pci_dev *pdev)
> > +{
> > +	u8 pin;
> > +	int irq;
> > +
> > +	irq = pdev->irq;
> > +	if (irq < 16)
> > +		return irq; /* ISA IRQ */
> > +
> > +	pin = pdev->pin;
> > +
> > +	/* We don't know the GSI. Specify the PCI INTx line instead. */
> > +	return ((uint64_t)0x01 << 56) | /* PCI INTx identifier */
> 
> You can use HVM_CALLBACK_VIA_TYPE_SHIFT here.

OK


> 
> > +		((uint64_t)pci_domain_nr(pdev->bus) << 32) |
> > +		((uint64_t)pdev->bus->number << 16) |
> > +		((uint64_t)(pdev->devfn & 0xff) << 8) |
> > +		((uint64_t)(pin - 1) & 3);
> > +}
> > +
> >
> 
>