linux-kernel - Re: [PATCH 9/9] qspinlock,x86,kvm: Implement KVM support for paravirt qspinlock

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Message-ID: <20150319100121.GL21418@twins.programming.kicks-ass.net>
Date:	Thu, 19 Mar 2015 11:01:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	paolo.bonzini@...il.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
	riel@...hat.com, torvalds@...ux-foundation.org,
	raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
	oleg@...hat.com, scott.norton@...com, doug.hatch@...com,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	luto@...capital.net
Subject: Re: [PATCH 9/9] qspinlock,x86,kvm: Implement KVM support for
 paravirt qspinlock

On Wed, Mar 18, 2015 at 10:45:55PM -0400, Waiman Long wrote:
> On 03/16/2015 09:16 AM, Peter Zijlstra wrote:
> I do have some concern about this call site patching mechanism as the
> modification is not atomic. The spin_unlock() calls are in many places in
> the kernel. There is a possibility that a thread is calling a certain
> spin_unlock call site while it is being patched by another one with the
> alternative() function call.
> 
> So far, I don't see any problem with bare metal where paravirt_patch_insns()
> is used to patch it to the move instruction. However, in a virtual guest
> enivornment where paravirt_patch_call() was used, there were situations
> where the system panic because of page fault on some invalid memory in the
> kthread. If you look at the paravirt_patch_call(), you will see:
> 
>     :
> b->opcode = 0xe8; /* call */
> b->delta = delta;
> 
> If another CPU reads the instruction at the call site at the right moment,
> it will get the modified call instruction, but not the new delta value. It
> will then jump to a random location. I believe that was causing the system
> panic that I saw.
> 
> So I think it is kind of risky to use it here unless we can guarantee that
> call site patching is atomic wrt other CPUs.

Just look at where the patching is done:

init/main.c:start_kernel()
  check_bugs()
    alternative_instructions()
      apply_paravirt()

We're UP and not holding any locks, disable IRQs (see text_poke_early())
and have NMIs 'disabled'.
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