linux-kernel - Re: [PATCH] apic: fix kvm build on UP without IOAPIC

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Message-ID: <20120709092436.GD2901@gmail.com>
Date:	Mon, 9 Jul 2012 11:24:36 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Stephen Rothwell <sfr@...b.auug.org.au>, x86@...nel.org,
	kvm@...r.kernel.org, linux-next@...r.kernel.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] apic: fix kvm build on UP without IOAPIC


* Michael S. Tsirkin <mst@...hat.com> wrote:

> On Fri, Jul 06, 2012 at 04:12:23PM +0200, Ingo Molnar wrote:
> > 
> > * Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > 
> > > On Fri, Jul 06, 2012 at 01:13:14PM +0200, Ingo Molnar wrote:
> > > > 
> > > > * H. Peter Anvin <hpa@...or.com> wrote:
> > > > 
> > > > > On 07/01/2012 08:05 AM, Michael S. Tsirkin wrote:
> > > > > >On UP i386, when APIC is disabled
> > > > > ># CONFIG_X86_UP_APIC is not set
> > > > > ># CONFIG_PCI_IOAPIC is not set
> > > > > >
> > > > > >code looking at apicdrivers never has any effect but it
> > > > > >still gets compiled in. In particular, this causes
> > > > > >build failures with kvm, but it generally bloats the kernel
> > > > > >unnecessarily.
> > > > > >
> > > > > >Fix by defining both __apicdrivers and __apicdrivers_end
> > > > > >to be NULL when CONFIG_X86_LOCAL_APIC is unset: I verified
> > > > > >that as the result any loop scanning __apicdrivers gets optimized out by
> > > > > >the compiler.
> > > > > >
> > > > > >Warning: a .config with apic disabled doesn't seem to boot
> > > > > >for me (even without this patch). Still verifying why,
> > > > > >meanwhile this patch is compile-tested only.
> > > > > >
> > > > > >Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > > > >---
> > > > > >
> > > > > >Note: if this patch makes sense, can x86 maintainers
> > > > > >please ACK applying it through the kvm tree, since that is
> > > > > >where we see the issue that it addresses?
> > > > > >Avi, Marcelo, maybe you can carry this in kvm/linux-next as a temporary
> > > > > >measure so that linux-next builds?
> > > > > >
> > > > > 
> > > > > I'm not happy about that as a workflow, but since you guys have an
> > > > > immediate problem I guess we can do that.
> > > > 
> > > > I'm rather unhappy about this workflow - we've got quite a few 
> > > > apic bits in the x86 tree this cycle as well and need extra 
> > > > external interaction, not.
> > > > 
> > > > Which KVM tree commit caused this, could someone please give a 
> > > > lkml link or quote it here? It's not referenced in the fix patch 
> > > > either.
> > > > 
> > > > Thanks,
> > > > 
> > > > 	Ingo
> > > 
> > > This tree (kvm.git next):
> > > 
> > > http://git.kernel.org/?p=virt/kvm/kvm.git;a=shortlog;h=refs/heads/next
> > > 
> > > Introduced by this commit:
> > > 
> > > http://git.kernel.org/?p=virt/kvm/kvm.git;a=commit;h=ab9cf4996bb989983e73da894b8dd0239aa2c3c2
> > 
> > This bit:
> > 
> > > +	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI)) {
> > > +		struct apic **drv;
> > > +
> > > +		for (drv = __apicdrivers; drv < __apicdrivers_end; drv++) {
> > > +			/* Should happen once for each apic */
> > > +			WARN_ON((*drv)->eoi_write == kvm_guest_apic_eoi_write);
> > > +			(*drv)->eoi_write = kvm_guest_apic_eoi_write;
> > > +		}
> > > +	}
> > > +
> > 
> > is rather disgusting I have to say.
> > 
> > WTH is the KVM code meddling with core x86 apic driver data 
> > structures directly? At minimum factor this out and create a 
> > proper apic.c function which is EXPORT_SYMBOL_GPL() exported or 
> > so...
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> OK, so apic_set_eoi_write()?

Yes, with a changelog comment analyzing the design decisions and 
locking here - what happens if actual APIC driver use races with 
this update on SMP, why is it all safe, etc?

Thanks,

	Ingo
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