Re: KVM: x86: Push potential exception error code on task switches

[Jan Kiszka <jan.kiszka@siemens.com>]

Gleb Natapov wrote:
> On Wed, Apr 14, 2010 at 02:11:39PM +0200, Jan Kiszka wrote:
>>  static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
>> -				    struct x86_emulate_ops *ops,
>> -				    u16 tss_selector, int reason)
>> +				   struct x86_emulate_ops *ops,
>> +				   u16 tss_selector, int reason,
>> +				   bool has_error_code, u32 error_code)
>>  {
>>  	struct desc_struct curr_tss_desc, next_tss_desc;
>>  	int ret;
>> @@ -2416,12 +2417,23 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
>>  	ops->set_cached_descriptor(&next_tss_desc, VCPU_SREG_TR, ctxt->vcpu);
>>  	ops->set_segment_selector(tss_selector, VCPU_SREG_TR, ctxt->vcpu);
>>  
>> +	if (ret == X86EMUL_CONTINUE && has_error_code) {
> It looks like we shouldn't get here if ret != X86EMUL_CONTINUE in the
> first place. This check should be done just after call to
> task_switch_16/32. Not directly related to your patch, but still...

Will do this in a preparational patch.

> 
>> @@ -2416,12 +2417,23 @@ static int emulator_do_task_switch(struct
>> x86_emulate_ctxt *ctxt,
>>       ops->set_cached_descriptor(&next_tss_desc, VCPU_SREG_TR,
>>       ctxt->vcpu);
>>       ops->set_segment_selector(tss_selector, VCPU_SREG_TR,
>>       ctxt->vcpu);
>>  
>> +     if (ret == X86EMUL_CONTINUE && has_error_code) {
>> +             struct decode_cache *c = &ctxt->decode;
>> +
>> +             c->op_bytes = c->ad_bytes = (next_tss_desc.type & 8) ? 4
>> : 2;
>> +             c->lock_prefix = 0;
>> +             c->src.val = (unsigned long) error_code;
>> +             emulate_push(ctxt);
>> +             ret = writeback(ctxt, ops);
>> +     }
> I would move writeback() to emulator_task_switch(). Just make
> c->dst.type = OP_NONE if writeback is not needed.

I should dramatically increase the overhead for the common case. :)
Yeah, can do so if preferred.

Thanks,
Jan

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