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

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Message-ID: <CAKLkAJ4=eT7TdZoOkQKMnhmrESq5kvCQL3Mw52a35gu2yWsNgw@mail.gmail.com>
Date:	Sun, 8 Jan 2012 10:47:49 +0200
From:	Nadav Amit <nadav.amit@...il.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...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

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.

Regards,
Nadav
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