Messages in this thread | | | From | Saurabh Singh Sengar <> | Subject | RE: [EXTERNAL] Re: [PATCH] x86/ioapic: Don't return 0 as valid virq | Date | Tue, 14 Mar 2023 10:23:40 +0000 |
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> -----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.
> > 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. > > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpeopl > e.kernel.org%2Ftglx%2Fnotes-about- > netiquette&data=05%7C01%7Cssengar%40microsoft.com%7C817c78e7bb324 > 8cd73b708db23b41c2a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0% > 7C638143028755917117%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjA > wMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C > %7C&sdata=3N5Mkl2gjMPHKOJGykZ3LvM6h%2FfD86dXLTQo3VH0Svc%3D&re > served=0
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