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

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

I just see mail to rahul.tanwar@linux.intel.com is undelivered, shall I still add it in "Cc:" ?
Please let me know what we usually do in such cases.

Regards,
Saurabh


> -----Original Message-----
> From: Saurabh Singh Sengar <ssengar@microsoft.com>
> Sent: Monday, March 13, 2023 9:00 AM
> To: Borislav Petkov <bp@alien8.de>; Saurabh Sengar
> <ssengar@linux.microsoft.com>
> Cc: 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
> 
> Cc: rahul.tanwar@linux.intel.com, andriy.shevchenko@intel.com
> 
> Thanks for you comments, please see my responses below.
> 
> > -----Original Message-----
> > From: Borislav Petkov <bp@alien8.de>
> > Sent: Monday, March 13, 2023 2:10 AM
> > To: Saurabh Sengar <ssengar@linux.microsoft.com>
> > Cc: 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
> > Subject: [EXTERNAL] Re: [PATCH] x86/ioapic: Don't return 0 as valid
> > virq
> >
> > On Thu, Mar 02, 2023 at 11:34:46AM -0800, Saurabh Sengar wrote:
> > > Zero is invalid virq and should't be returned as a valid value for
> > > lower irq bound. If IO-APIC and gsi_top are not initialized return
> >
> > Why isn't gsi_top initialized?
> >
> > What is this fixing?
> 
> In the absence of a device tree node for IO-APIC,  IO-APIC is not registered,
> resulting in uninitialized gsi_top. And in such cases arch_dynirq_lower_bound
> will return 0. Returning 0 from this function will allow interrupts to have 0
> assigned as valid irq, which is wrong. In case gsi_top is 0, lower bound of irq
> should be derived from 'hint' value passed to function as 'from'.
> 
> I can add above info in commit message,  please let me know if anything
> more to be added.
> 
> 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.
> 
> >
> > Don't be afraid to do
> >
> > git annotate arch/x86/kernel/apic/io_apic.c
> >
> > and see which commit added this. This one:
> >
> > 3e5bedc2c258 ("x86/apic: Fix arch_dynirq_lower_bound() bug for DT
> > enabled
> > machines")
> >
> > Now add the folks from this commit to Cc and tell them why in your
> > case gsi_top is not initialized and what they're breaking by doing that.
> 
> Thanks. I will add "Fixes:" and "Cc:" tag in next version.
> 
> >
> > The more your commit message explains *why* you're fixing something,
> > the better it is for the maintainers/reviewers to actually know what to do.
> >
> > Right now I'm reading this and I'm thinking, random, unjustified change.
> > Ignore.
> >
> > Ok?
> >
> > Thx.
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> >
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