linux-kernel - Re: [PATCH] KVM: fix mov immediate emulation for 64-bit operands

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Date:	Sun, 8 Jan 2012 10:26:57 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Nadav Amit <nadav.amit@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: fix mov immediate emulation for 64-bit operands

Hi,

Nadav Amit <nadav.amit@...il.com> wrote: 
> On Jan 7, 2012, at 10:25 PM, H. Peter Anvin wrote:
> 
> > On 01/07/2012 12:21 PM, Nadav Amit wrote:
> >> MOV immediate instruction (opcodes 0xB8-0xBF) may take 64-bit operand.
> >> The previous emulation implementation assumes the operand is no longer than 32.
> >> 
> >> Signed-off-by: Nadav Amit <nadav.amit@...il.com>
> > 
> > There are exactly two such instructions: MOV immediate (B8-BF) and MOV
> > moff (A0-A3); you may want to check the latter too.
> > 
> > 	-hpa
> > 
> 
> These instructions (A0-A3) seem to be already covered by the decode_abs function.

Like these how about introducing a new flag and change the following entries in the
decode table to indicate possible 64bit immediate:

	/* 0xB8 - 0xBF */
	X8(I(DstReg | SrcImm | Mov, em_mov)),	

Checking the opcode byte at the operand decoding stage, like below, does not look nice:
	(IMO so better ask Avi)

+	if (size == 8 && ((ctxt->b & 0xF8) != 0xB8 || ctxt->twobyte))
		size = 4;

I am now cleaning up x86_decode_insn() to make each decoding stage clearer.

	Takuya
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