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

[Paolo Bonzini]

On 03/11/2015 15:02, Laszlo Ersek wrote:
> On 11/03/15 14:46, Paolo Bonzini wrote:
>>
>>
>> On 03/11/2015 14:40, Laszlo Ersek wrote:
>>> 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 KrÄmÃÅ <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?
>>
>> Just use whatever you were using before, and revert commit c9db607
>> ("UefiCpuPkg: PiSmmCpuDxeSmm: do not execute RSM from 64-bit mode",
>> 2015-10-14) from your OVMF branch.
> 
> Right, I planned to do that OVMF-side revert; I just wasn't sure if e.g.
> kvm/queue had some prerequisite patches for this.

Indeed, you can use either your "part 2" series or Radim's patches from
kvm/queue, it's the same.

Paolo
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