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From: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Denis Lunev" <den@virtuozzo.com>,
	"Roman Kagan" <rkagan@virtuozzo.com>,
	"Denis Plotnikov" <dplotnikov@virtuozzo.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Jim Mattson" <jmattson@google.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Yi Wang" <wang.yi59@zte.com.cn>,
	"Peng Hao" <peng.hao2@zte.com.cn>
Subject: Re: [PATCH 3/3] KVM: x86: always stop emulation on page fault
Date: Wed, 28 Aug 2019 10:19:51 +0000	[thread overview]
Message-ID: <20190828131948.cb67f97cab502b9f5f63b1b8@virtuozzo.com> (raw)
In-Reply-To: <20190827145030.GC27459@linux.intel.com>

On Tue, 27 Aug 2019 07:50:30 -0700
Sean Christopherson <sean.j.christopherson@intel.com> wrote:

> +Cc Peng Hao and Yi Wang
> 
> On Tue, Aug 27, 2019 at 01:07:09PM +0000, Jan Dakinevich wrote:
> > inject_emulated_exception() returns true if and only if nested page
> > fault happens. However, page fault can come from guest page tables
> > walk, either nested or not nested. In both cases we should stop an
> > attempt to read under RIP and give guest to step over its own page
> > fault handler.
> > 
> > Fixes: 6ea6e84 ("KVM: x86: inject exceptions produced by x86_decode_insn")
> > Cc: Denis Lunev <den@virtuozzo.com>
> > Cc: Roman Kagan <rkagan@virtuozzo.com>
> > Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
> > Signed-off-by: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
> > ---
> >  arch/x86/kvm/x86.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 93b0bd4..45caa69 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6521,8 +6521,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> >  			if (reexecute_instruction(vcpu, cr2, write_fault_to_spt,
> >  						emulation_type))
> >  				return EMULATE_DONE;
> > -			if (ctxt->have_exception && inject_emulated_exception(vcpu))
> > +			if (ctxt->have_exception) {
> > +				inject_emulated_exception(vcpu);
> >  				return EMULATE_DONE;
> > +			}
> 
> 
> Yikes, this patch and the previous have quite the sordid history.
> 
> 
> The non-void return from inject_emulated_exception() was added by commit
> 
>   ef54bcfeea6c ("KVM: x86: skip writeback on injection of nested exception")
> 
> for the purpose of skipping writeback.  At the time, the above blob in the
> decode flow didn't exist.
> 
> 
> Decode exception handling was added by commit
> 
>   6ea6e84309ca ("KVM: x86: inject exceptions produced by x86_decode_insn")
> 
> but it was dead code even then.  The patch discussion[1] even point out that
> it was dead code, i.e. the change probably should have been reverted.
> 
> 
> Peng Hao and Yi Wang later ran into what appears to be the same bug you're
> hitting[2][3], and even had patches temporarily queued[4][5], but the
> patches never made it to mainline as they broke kvm-unit-tests.  Fun side
> note, Radim even pointed out[4] the bug fixed by patch 1/3.
> 
> So, the patches look correct, but there's the open question of why the
> hypercall test was failing for Paolo.  

Sorry, I'm little confused. Could you please, point me which test or tests 
were broken? I've just run kvm-unit-test and I see same results with and 
without my changes.

> I've tried to reproduce the #DF to
> no avail.
> 
> [1] https://lore.kernel.org/patchwork/patch/850077/
> [2] https://lkml.kernel.org/r/1537311828-4547-1-git-send-email-penghao122@sina.com.cn
> [3] https://lkml.kernel.org/r/20190111133002.GA14852@flask
> [4] https://lkml.kernel.org/r/20190111133002.GA14852@flask
> [5] https://lkml.kernel.org/r/9835d255-dd9a-222b-f4a2-93611175b326@redhat.com
> 
> >  			if (emulation_type & EMULTYPE_SKIP)
> >  				return EMULATE_FAIL;
> >  			return handle_emulation_failure(vcpu, emulation_type);
> > -- 
> > 2.1.4
> > 


-- 
Best regards
Jan Dakinevich

  parent reply	other threads:[~2019-08-28 10:19 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-27 13:07 [PATCH 0/3] fix emulation error on Windows bootup Jan Dakinevich
2019-08-27 13:07 ` [PATCH 1/3] KVM: x86: fix wrong return code Jan Dakinevich
2019-08-27 13:42   ` Sean Christopherson
2019-08-27 13:07 ` [PATCH 2/3] KVM: x86: set ctxt->have_exception in x86_decode_insn() Jan Dakinevich
2019-08-27 14:53   ` Sean Christopherson
2019-08-28 10:19     ` Jan Dakinevich
2019-08-27 13:07 ` [PATCH 3/3] KVM: x86: always stop emulation on page fault Jan Dakinevich
2019-08-27 14:50   ` Sean Christopherson
2019-08-27 14:55     ` Sean Christopherson
2019-08-28 10:19     ` Jan Dakinevich [this message]
2019-08-28 14:23       ` Sean Christopherson
2019-09-11 15:51 ` [PATCH 0/3] fix emulation error on Windows bootup Paolo Bonzini
2019-09-11 19:53   ` Sean Christopherson
2019-09-13  9:51     ` Paolo Bonzini

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