linux-kernel - Re: [PATCH 1/2] KVM: Exception during emulation decode should propagate

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Message-ID: <4F0E28C7.2010409@oss.ntt.co.jp>
Date:	Thu, 12 Jan 2012 09:26:47 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
CC:	Nadav Amit <namit@...technion.ac.il>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH 1/2] KVM: Exception during emulation decode should propagate

(2012/01/12 7:11), Takuya Yoshikawa wrote:
> On Wed, 11 Jan 2012 18:53:30 +0200
> Nadav Amit<namit@...technion.ac.il>  wrote:
>
>> An exception might occur during decode (e.g., #PF during fetch).
>> Currently, the exception is ignored and emulation is performed.

Note that the decode/emulation will not be continued in such a case.

insn_fetch() is a bit tricky macro and it contains "goto done" to outside.
So if an error happens during fetching the instruction, x86_decode_insn()
will handle the X86EMUL_* fault value and returns FAIL immediately.

	Takuya

>
> When I cleaned up insn_fetch(), I thought that fetching the instruction
> which is being executed by the guest cannot cause #PF.
>
> The possibility that a meaningless userspace might similtaneously unmap
> the page, noted by Avi IIRC, was ignored intentionally, so we just fail
> in such a case.
>
> Did you see any real problem?
>
> 	Takuya
>
>
>> Instead, emulation should be skipped and the fault should be injected.
>> Skipping instruction should report a failure in this case.
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