Re: [PATCH 3/3] KVM: x86 emulator: Allow PM/VM86 switch during task switch

[Kevin Wolf <kwolf@redhat.com>]

Am 24.01.2012 11:57, schrieb Gleb Natapov:
> On Mon, Jan 23, 2012 at 05:10:48PM +0100, Kevin Wolf wrote:
>> Task switches can switch between Protected Mode and VM86. The current
>> mode must be updated during the task switch emulation so that the new
>> segment selectors are interpreted correctly and privilege checks
>> succeed.
>>
>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>> ---
>>  arch/x86/include/asm/kvm_emulate.h |    1 +
>>  arch/x86/kvm/emulate.c             |   17 +++++++++++++++++
>>  arch/x86/kvm/x86.c                 |    6 ++++++
>>  3 files changed, 24 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
>> index c8a9cf3..4a21c7d 100644
>> --- a/arch/x86/include/asm/kvm_emulate.h
>> +++ b/arch/x86/include/asm/kvm_emulate.h
>> @@ -176,6 +176,7 @@ struct x86_emulate_ops {
>>  	void (*set_idt)(struct x86_emulate_ctxt *ctxt, struct desc_ptr *dt);
>>  	ulong (*get_cr)(struct x86_emulate_ctxt *ctxt, int cr);
>>  	int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val);
>> +	void (*set_rflags)(struct x86_emulate_ctxt *ctxt, ulong val);
>>  	int (*cpl)(struct x86_emulate_ctxt *ctxt);
>>  	int (*get_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong *dest);
>>  	int (*set_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong value);
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 833969e..52fce89 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -2273,6 +2273,23 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
>>  		return emulate_gp(ctxt, 0);
>>  	ctxt->_eip = tss->eip;
>>  	ctxt->eflags = tss->eflags | 2;
>> +
>> +	/*
>> +	 * If we're switching between Protected Mode and VM86, we need to make
>> +	 * sure to update the mode before loading the segment descriptors so
>> +	 * that the selectors are interpreted correctly.
>> +	 *
>> +	 * Need to get it to the vcpu struct immediately because it influences
>> +	 * the CPL which is checked when loading the segment descriptors.
> This is true only for VMX. SVM does not look at rflags. May be instead
> of adding new x86_emulate_ops callback we need to get rid of get_cpl()
> one and implement CPL checking using emulate.c:get_segment_selector().

The selector isn't enough for VM86. In most cases
ctxt->ops->get_segment() would work (assuming that the dpl field is
valid there), but it doesn't in task switches before switching the code
segment, which already needs to have the new CPL applied to succeed.

So the only other way I could think of is to pass a flag to
load_segment_descriptor() which says that it shouldn't check any privileges.

Kevin