[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at) instead of open-coding an IRQ allocator.
On 10/28/2010 05:43 AM, Stefano Stabellini wrote: > On Tue, 26 Oct 2010, Konrad Rzeszutek Wilk wrote: >>> In that case we should use dynamic allocation for everything. Or try to >>> work out distinct irq ranges for different interrupts if you really want >>> to keep irq==gsi. >> Some little alarm bells are ringing in the back of my head about irq != gsi. >> >> I think the issue was the permission. When a PCI device is allocated to the >> PV guest, we do a bunch of xc_* calls to allow the domain to use the BARs >> and the IRQ. I believe when the guest boots and tries to map the >> event channel with the physical IRQ, one of the arguments is that GSI. And >> if we provide a bogus GSI, well, we won't get the INTx to the guest. >> >> As you mentioned, Stefano's patch add a new element to the tuple that can >> contain the GSI value. At which point we can make the guest IRQ != GSI, >> as long as we can contain the <gsi, event channel> mapping present so >> that for the hypercalls we can give it the right GSI. >> >> The MSI/MSI-X use a completly different mechanism that does not all >> of this complication, so we are OK with that. >> >> .. snip .. >> >>> d) dynamically allocate all irqs for all event channel types. >> <nods> Ok, you sold me on this idea. >> > > Even though dynamic allocation might seem possible for both pirqs and > irqs, there are some severe limitations: > > > - Xen won't allocate pirq numbers lower than 16 (probably because it > expects pirq == gsi for the first 16 gsi), so it might run out > of pirqs if we ask Xen to always choose the pirq number for us. As a > consequence it is safer to keep using pirq == gsi, at least for the > first 16 gsis. This limitation should probably be fixed in Xen, but we > need to support older hypervisors so we cannot rely on the fix to be > present. > > > - Linux expects irq == gsi, see arch/x86/kernel/acpi/boot.c:gsi_to_irq > > /* Provide an identity mapping of gsi == irq > * except on truly weird platforms that have > * non isa irqs in the first 16 gsis. > */ Yes, we always have to identity map legacy ISA interrupts, and we should never attempt to dynamically allocate in that region. J _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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