linux-kernel - Re: [PATCH] KVM: x86: MOV CR/DR emulation should ignore mod

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Date:	Mon, 26 May 2014 14:17:28 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Nadav Amit <namit@...technion.ac.il>
CC:	gleb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: MOV CR/DR emulation should ignore mod

Il 25/05/2014 22:05, Nadav Amit ha scritto:
> MOV CR/DR instructions ignore the mod field (in the ModR/M byte). As the SDM
> states: "The 2 bits in the mod field are ignored".  Accordingly, the second
> operand of these instructions is always a general purpose register.
>
> The current emulator implementation does not do so. If the mod bits do not
> equal 3, it expects the second operand to be in memory.
>
> Signed-off-by: Nadav Amit <namit@...technion.ac.il>
> ---
>  arch/x86/kvm/emulate.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 2fa7ab0..e4e833d 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -161,6 +161,7 @@
>  #define Fastop      ((u64)1 << 44)  /* Use opcode::u.fastop */
>  #define NoWrite     ((u64)1 << 45)  /* No writeback */
>  #define SrcWrite    ((u64)1 << 46)  /* Write back src operand */
> +#define NoMod	    ((u64)1 << 47)  /* Mod field is ignored */
>
>  #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
>
> @@ -1077,7 +1078,7 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
>  	ctxt->modrm_rm |= (ctxt->modrm & 0x07);
>  	ctxt->modrm_seg = VCPU_SREG_DS;
>
> -	if (ctxt->modrm_mod == 3) {
> +	if (ctxt->modrm_mod == 3 || (ctxt->d & NoMod)) {
>  		op->type = OP_REG;
>  		op->bytes = (ctxt->d & ByteOp) ? 1 : ctxt->op_bytes;
>  		op->addr.reg = decode_register(ctxt, ctxt->modrm_rm,
> @@ -3877,10 +3878,12 @@ static const struct opcode twobyte_table[256] = {
>  	N, N, N, N, N, N, N, N,
>  	D(ImplicitOps | ModRM), N, N, N, N, N, N, D(ImplicitOps | ModRM),
>  	/* 0x20 - 0x2F */
> -	DIP(ModRM | DstMem | Priv | Op3264, cr_read, check_cr_read),
> -	DIP(ModRM | DstMem | Priv | Op3264, dr_read, check_dr_read),
> -	IIP(ModRM | SrcMem | Priv | Op3264, em_cr_write, cr_write, check_cr_write),
> -	IIP(ModRM | SrcMem | Priv | Op3264, em_dr_write, dr_write, check_dr_write),
> +	DIP(ModRM | DstMem | Priv | Op3264 | NoMod, cr_read, check_cr_read),
> +	DIP(ModRM | DstMem | Priv | Op3264 | NoMod, dr_read, check_dr_read),
> +	IIP(ModRM | SrcMem | Priv | Op3264 | NoMod, em_cr_write, cr_write,
> +						check_cr_write),
> +	IIP(ModRM | SrcMem | Priv | Op3264 | NoMod, em_dr_write, dr_write,
> +						check_dr_write),
>  	N, N, N, N,
>  	GP(ModRM | DstReg | SrcMem | Mov | Sse, &pfx_0f_28_0f_29),
>  	GP(ModRM | DstMem | SrcReg | Mov | Sse, &pfx_0f_28_0f_29),
>

This is easy to test in kvm-unit-tests.  Please provide a patch for that 
as well. :)

Paolo
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