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

[Andre Przywara <andre.przywara@xxxxxxx>]

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 
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?
Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12


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