Re: [PATCH] x86/ioapic: Don't return 0 as valid virq

[Saurabh Singh Sengar <ssengar@microsoft.com>]

> -----Original Message-----
> From: Saurabh Singh Sengar <ssengar@microsoft.com>
> Sent: Tuesday, March 14, 2023 3:54 PM
> To: Borislav Petkov <bp@alien8.de>
> Cc: Saurabh Sengar <ssengar@linux.microsoft.com>; tglx@linutronix.de;
> mingo@redhat.com; dave.hansen@linux.intel.com; x86@kernel.org;
> hpa@zytor.com; johan+linaro@kernel.org; isaku.yamahata@intel.com;
> Michael Kelley (LINUX) <mikelley@microsoft.com>; linux-
> kernel@vger.kernel.org; rahul.tanwar@linux.intel.com;
> andriy.shevchenko@intel.com
> Subject: RE: [EXTERNAL] Re: [PATCH] x86/ioapic: Don't return 0 as valid virq
> 
> 
> 
> > -----Original Message-----
> > From: Borislav Petkov <bp@alien8.de>
> > Sent: Monday, March 13, 2023 4:44 PM
> > To: Saurabh Singh Sengar <ssengar@microsoft.com>
> > Cc: Saurabh Sengar <ssengar@linux.microsoft.com>; tglx@linutronix.de;
> > mingo@redhat.com; dave.hansen@linux.intel.com; x86@kernel.org;
> > hpa@zytor.com; johan+linaro@kernel.org; isaku.yamahata@intel.com;
> > Michael Kelley (LINUX) <mikelley@microsoft.com>; linux-
> > kernel@vger.kernel.org; rahul.tanwar@linux.intel.com;
> > andriy.shevchenko@intel.com
> > Subject: Re: [EXTERNAL] Re: [PATCH] x86/ioapic: Don't return 0 as valid virq
> >
> > On Mon, Mar 13, 2023 at 03:29:32AM +0000, Saurabh Singh Sengar wrote:
> > > To be specific in our system which is a guest VM we don't need IO-APIC
> > > and hence there is no device tree node for it. It is observed that we get irq
> 0
> > assigned to PCI-MSI.
> >
> > This should be added to your commit message: what guest VM is that and
> > why should the kernel support it.
> 
> Guest VM is a linux VM running as child partition on Hyper-V. Hyper-v Linux
> documentation is in Documentation/virt/hyperv/.
> 
> In commit I wanted to mention that any system which is not registering IO-
> APIC
> will have this issue. But I am fine to mention specifically about the issue I am
> facing.
> As part of your next comment, I have explained the issue in detail if that is
> good, I
> can put that as commit message.
> 
> >
> > Why doesn't it need an IO-APIC and why does the current code need to be
> > changed just for your guest VM?
> 
> For Hyper-V Virtual Machines, few platforms don't have any devices to be
> hooked to IO-APIC. Although it has Hyper-V based MSI over VMBus which
> assigns interrupts to PCIe devices. In such platforms IO-APIC is not
> registered which causes gsi_top value to remain at 0 and not get properly
> assigned. Moreover, due to the inability to disable CONFIG_X86_IO_APIC
> flag, the io-apic code still gets compiled. Thus, arch_dynirq_lower_bound
> function in io_apic.c decides the lower bound of irq numbers based on
> gsi_top.
> 
> Later when PCIe-MSI attempts to allocate interrupts, it gets 0 as the first
> virq number because gsi_top is still 0. 0 being invalid virq is ignored by
> MSI irq domain and results allocation of the same PCIe MSI twice.
> 
> 		CPU0		CPU1
> 0:		2			0		Hyper-V PCIe MSI
> 1073741824-edge
> 1:		69			0		Hyper-V PCIe MSI
> 1073741824-edge      nvme0q0
> 
> To avoid this issue, if IO-APIC and gsi_top are not initialized, return the
> hint value passed as 'from' value to arch_dynirq_lower_bound instead of 0.
> This will also be identical to the behaviour of weak arch_dynirq_lower_bound
> function defined in kernel/softirq.c.

Hi Borislav,

Have you had an opportunity to review the above commit message ?
Kindly provide me with your opinion on whether it meets your expectations.

Regards,
Saurabh

> 
> >
> > What else needs to be changed so that your VM works?
> 
> This is the only change required.
> 
> >
> > Where is that VM's documentation and why can't that VM be fixed *not* to
> > need kernel changes? IOW, why can't that VM emulate an IO-APIC like the
> > others do...
> 
> Documentation is mentioned above. As there is no need of IO-APIC there is
> no need emulating it.
> 
> Please let me know if there is any further clarification required.
> 
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> >
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