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Message-ID: <d55ccf1c085d4adadc8dbbbae6443059a94eaf90.camel@linux.intel.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: Fix wrong gfn range of tlb
 flushing in validate_direct_spte()
From:   Robert Hoo <robert.hu@linux.intel.com>
To:     David Matlack <dmatlack@google.com>
Cc:     Hou Wenlong <houwenlong.hwl@antgroup.com>,
        kvm list <kvm@vger.kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Lan Tianyu <Tianyu.Lan@microsoft.com>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Tue, 27 Sep 2022 10:54:10 +0800
In-Reply-To: <CALzav=f=y7-2uOnXUi---hvCTa2otDBPsY1VoUtDWnS7+0QX=w@mail.gmail.com>
References: <cover.1661331396.git.houwenlong.hwl@antgroup.com>
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         <f6fd8ccff13f9f48cbca06f0a5278654198d0d06.camel@linux.intel.com>
         <YyoHNMz3CH4SnJwJ@google.com>
         <CALzav=f=y7-2uOnXUi---hvCTa2otDBPsY1VoUtDWnS7+0QX=w@mail.gmail.com>
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On Tue, 2022-09-20 at 11:44 -0700, David Matlack wrote:
> On Tue, Sep 20, 2022 at 11:32 AM David Matlack <dmatlack@google.com>
> wrote:
> > 
> > On Sun, Sep 18, 2022 at 09:11:00PM +0800, Robert Hoo wrote:
> > > On Wed, 2022-08-24 at 17:29 +0800, Hou Wenlong wrote:
> > > > The spte pointing to the children SP is dropped, so the
> > > > whole gfn range covered by the children SP should be flushed.
> > > > Although, Hyper-V may treat a 1-page flush the same if the
> > > > address points to a huge page, it still would be better
> > > > to use the correct size of huge page. Also introduce
> > > > a helper function to do range-based flushing when a direct
> > > > SP is dropped, which would help prevent future buggy use
> > > > of kvm_flush_remote_tlbs_with_address() in such case.
> > > > 
> > > > Fixes: c3134ce240eed ("KVM: Replace old tlb flush function with
> > > > new
> > > > one to flush a specified range.")
> > > > Suggested-by: David Matlack <dmatlack@google.com>
> > > > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> > > > ---
> > > >  arch/x86/kvm/mmu/mmu.c | 10 +++++++++-
> > > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > index e418ef3ecfcb..a3578abd8bbc 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -260,6 +260,14 @@ void
> > > > kvm_flush_remote_tlbs_with_address(struct
> > > > kvm *kvm,
> > > >     kvm_flush_remote_tlbs_with_range(kvm, &range);
> > > >  }
> > > > 
> > > > +/* Flush all memory mapped by the given direct SP. */
> > > > +static void kvm_flush_remote_tlbs_direct_sp(struct kvm *kvm,
> > > > struct
> > > > kvm_mmu_page *sp)
> > > > +{
> > > > +   WARN_ON_ONCE(!sp->role.direct);
> > > 
> > > What if !sp->role.direct? Below flushing sp->gfn isn't expected?
> > > but
> > > still to do it. Is this operation harmless?
> > 
> > Flushing TLBs is always harmless because KVM cannot ever assume an
> > entry is
> > in the TLB. However, *not* (properly) flushing TLBs can be harmful.
> > If KVM ever
> > calls kvm_flush_remote_tlbs_direct_sp() with an indirect SP, that
> > is a bug in
> > KVM. The TLB flush here won't be harmful, as I explained, but KVM
> > will miss a
> > TLB flush.
> > 
Yes, agree, not harmful, a cost of TLB miss, thanks.

> > That being said, I don't think any changes here are necessary.
> > kvm_flush_remote_tlbs_direct_sp() only has one caller,
> > validate_direct_spte(),
> > which only operates on direct SPs. The name of the function also
> > makes it
> > obvious this should only be called with a direct SP. And if we ever
> > mess this
> > up in the future, we'll see the WARN_ON().
> 
> That being said, we might as well replace the WARN_ON_ONCE() with
> KVM_BUG_ON(). That will still do a WARN_ON_ONCE() but has the added
> benefit of terminating the VM.

Yeah, here was my point, WARN_ON_ONCE() might not be warning obviously
enough, as it usually for recoverable cases. But terminating VM is also
over action I think. Just my 2 cents, the whole patch is good.