Re: [PATCH] KVM: x86: allow RSM from 64-bit mode

[Laszlo Ersek <lersek@redhat.com>]

On 11/03/15 14:29, Paolo Bonzini wrote:
> The SDM says that exiting system management mode from 64-bit mode
> is invalid, but that would be too good to be true.  But actually,
> most of the code is already there to support exiting from compat
> mode (EFER.LME=1, EFER.LMA=0).  Getting all the way from 64-bit
> mode to real mode only requires clearing CS.L and CR4.PCIDE.
> 
> Cc: stable@vger.kernel.org
> Fixes: 660a5d517aaab9187f93854425c4c63f4a09195c
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/emulate.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index b60fed56671b..1505587d06e9 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2484,16 +2484,36 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
>  
>  	/*
>  	 * Get back to real mode, to prepare a safe state in which to load
> -	 * CR0/CR3/CR4/EFER.
> -	 *
> -	 * CR4.PCIDE must be zero, because it is a 64-bit mode only feature.
> +	 * CR0/CR3/CR4/EFER.  It's all a bit more complicated if the vCPU
> +	 * supports long mode.
>  	 */
> +	cr4 = ctxt->ops->get_cr(ctxt, 4);
> +	if (emulator_has_longmode(ctxt)) {
> +		struct desc_struct cs_desc;
> +
> +		/* Zero CR4.PCIDE before CR0.PG.  */
> +		if (cr4 & X86_CR4_PCIDE) {
> +			ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE);
> +			cr4 &= ~X86_CR4_PCIDE;
> +		}
> +
> +		/* A 32-bit code segment is required to clear EFER.LMA.  */
> +		memset(&cs_desc, 0, sizeof(cs_desc));
> +		cs_desc.type = 0xb;
> +		cs_desc.s = cs_desc.g = cs_desc.p = 1;
> +		ctxt->ops->set_segment(ctxt, 0, &cs_desc, 0, VCPU_SREG_CS);
> +	}
> +
> +	/* For the 64-bit case, this will clear EFER.LMA.  */
>  	cr0 = ctxt->ops->get_cr(ctxt, 0);
>  	if (cr0 & X86_CR0_PE)
>  		ctxt->ops->set_cr(ctxt, 0, cr0 & ~(X86_CR0_PG | X86_CR0_PE));
> -	cr4 = ctxt->ops->get_cr(ctxt, 4);
> +
> +	/* Now clear CR4.PAE (which must be done before clearing EFER.LME).  */
>  	if (cr4 & X86_CR4_PAE)
>  		ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PAE);
> +
> +	/* And finally go back to 32-bit mode.  */
>  	efer = 0;
>  	ctxt->ops->set_msr(ctxt, MSR_EFER, efer);
>  
> @@ -4454,7 +4474,7 @@ static const struct opcode twobyte_table[256] = {
>  	F(DstMem | SrcReg | Src2CL | ModRM, em_shld), N, N,
>  	/* 0xA8 - 0xAF */
>  	I(Stack | Src2GS, em_push_sreg), I(Stack | Src2GS, em_pop_sreg),
> -	II(No64 | EmulateOnUD | ImplicitOps, em_rsm, rsm),
> +	II(EmulateOnUD | ImplicitOps, em_rsm, rsm),
>  	F(DstMem | SrcReg | ModRM | BitOp | Lock | PageTable, em_bts),
>  	F(DstMem | SrcReg | Src2ImmByte | ModRM, em_shrd),
>  	F(DstMem | SrcReg | Src2CL | ModRM, em_shrd),
> 

What branch should I test this on top of?

Thank you!
Laszlo