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@xxxxxxxxxxxxxxx
> Fixes: 660a5d517aaab9187f93854425c4c63f4a09195c
> Cc: Laszlo Ersek <lersek@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> 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
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