RE: [Xen-devel] [PATCH] x86/hvm: accelerate I/O intercept handling

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

Andre Przywara wrote:
> Cui, Dexuan wrote:
>> Actually in 15425:6e934c799051, VMX  once enabled the feature, but
>> IIRC, later Keir removed that in some cleanup patches and let the
>> EXIT_REASON_IO_INSTRUCTION handler invoke handle_mmio() also -- I
>> don't remember how people commented the slowness of IN/OUT emulation
>> caused the change...
> 
> If I got this correctly, 15425 introduces only the OUTS/INS segment
Oh... sorry, my memory was bad...

> decoding feature, something that SVM does not have (yet ;-)
> As I said in my mail, I doubt the usefulness of the shortcut in this
> case, since it would require to access the guest memory and is used
> very rarely. So for the sake of a saner implementation I would avoid
> utilizing this feature, it simplifies the code much and avoids code
> duplication. That's why my code just implements non-string
> instructions. 
> 
>> One thing is: IIRC, old Intel CPUs don't supply the info, but your
>  > patch doesn't check that... so your patch can break the CPUs.
>  > Please refer to 15425.
> But this is only true for the segment decoding part, right? Appendix G
> of the Intel 3B manual only speaks of _this_ feature protected by bit
> 54 of IA32_VMX_BASIC MSR. Is the information shown in table 23-5 in
> section 
> 23.2.1 not valid on all CPUs? If not, where can I check the support
> for this?
I mistook IN/OUT for INS/OUTS here. Sorry.
The patch looks good to me.

Thanks,
-- Dexuan


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