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

[Paolo Bonzini]

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.  This is how I tested it, in fact.

Paolo
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