From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Xiong Zhang <xiong.y.zhang@intel.com>,
Wayne Boyer <wayne.boyer@intel.com>,
Zhenyu Wang <zhenyuw@linux.intel.com>,
Jun Nakajima <jun.nakajima@intel.com>
Subject: Re: [PATCH] KVM: x86/mmu: Add capability to zap only sptes for the affected memslot
Date: Mon, 13 Jul 2020 12:06:50 -0700 [thread overview]
Message-ID: <20200713190649.GE29725@linux.intel.com> (raw)
In-Reply-To: <20200713122226.28188f93@x1.home>
On Mon, Jul 13, 2020 at 12:22:26PM -0600, Alex Williamson wrote:
> On Thu, 9 Jul 2020 21:29:22 -0700
> Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>
> > +Alex, whom I completely spaced on Cc'ing.
> >
> > Alex, this is related to the dreaded VFIO memslot zapping issue from last
> > year. Start of thread: https://patchwork.kernel.org/patch/11640719/.
> >
> > The TL;DR of below: can you try the attached patch with your reproducer
> > from the original bug[*]? I honestly don't know whether it has a legitimate
> > chance of working, but it's the one thing in all of this that I know was
> > definitely a bug. I'd like to test it out if only to sate my curiosity.
> > Absolutely no rush.
>
> Mixed results, maybe you can provide some guidance. Running this
> against v5.8-rc4, I haven't reproduced the glitch. But it's been a
> long time since I tested this previously, so I went back to v5.3-rc5 to
> make sure I still have a recipe to trigger it. I can still get the
> failure there as the selective flush commit was reverted in rc6. Then
> I wondered, can I take broken v5.3-rc5 and apply this fix to prove that
> it works? No, v5.3-rc5 + this patch still glitches. So I thought
> maybe I could make v5.8-rc4 break by s/true/false/ in this patch.
> Nope. Then I applied the original patch from[1] to try to break it.
> Nope. So if anything, I think the evidence suggests this was broken
> elsewhere and is now fixed, or maybe it is a timing issue that I can't
> trigger on newer kernels. If the reproducer wasn't so touchy and time
> consuming, I'd try to bisect, but I don't have that sort of bandwidth.
Ow. That manages to be both a best case and worst case scenario. I can't
think of any clever way to avoid bisecting. There have been a number of
fixes in tangentially related code since 5.3, e.g. memslots, MMU, TLB,
etc..., but trying to isolate which one, if any of them, fixed the bug has
a high probability of being a wild goose chase.
The only ideas I have going forward are to:
a) Reproduce the bug outside of your environment and find a resource that
can go through the painful bisection.
b) Add a module param to toggle the new behavior and see if anything
breaks.
I can ask internally if it's possible to get a resource on my end to go
after (a). (b) is a question for Paolo.
Thanks much for testing!
> Thanks,
>
> Alex
>
> [1] https://patchwork.kernel.org/patch/10798453/
>
next prev parent reply other threads:[~2020-07-13 19:06 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-03 2:50 [PATCH] KVM: x86/mmu: Add capability to zap only sptes for the affected memslot Sean Christopherson
2020-07-08 16:08 ` Paolo Bonzini
2020-07-09 21:12 ` Sean Christopherson
2020-07-09 22:18 ` Paolo Bonzini
2020-07-10 4:29 ` Sean Christopherson
2020-07-13 18:22 ` Alex Williamson
2020-07-13 19:06 ` Sean Christopherson [this message]
2020-07-21 3:03 ` Sean Christopherson
2020-07-21 16:00 ` Alex Williamson
2020-07-23 15:57 ` Sean Christopherson
2020-07-23 18:35 ` Alex Williamson
2020-09-14 16:49 ` Sean Christopherson
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