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

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Message-ID: <4F09A2A0.1060801@redhat.com>
Date:	Sun, 08 Jan 2012 16:05:20 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Nadav Amit <nadav.amit@...il.com>
CC:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>,
	"H. Peter Anvin" <hpa@...or.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

On 01/08/2012 10:47 AM, Nadav Amit wrote:
> On Sun, Jan 8, 2012 at 3:26 AM, Takuya Yoshikawa
> <takuya.yoshikawa@...il.com> wrote:
> > 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 agree. I remembered these flags are expensive (from the time flags
> were set in u32).
> I guess I can add OpImm64.
> Another less preferable alternative is to add a misc. flag or reuse
> another flag.
>
> Avi, please acknowledge adding OpImm64.

Yes, OpImm64 is the cleanest IMO.  Note it doesn't even cost us a bit.

-- 
error compiling committee.c: too many arguments to function

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