Re: [PATCH] KVM: x86: Introduce segmented_write_std

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-01-20 18:09+0100, Paolo Bonzini:
> On 20/01/2017 17:55, Jim Mattson wrote:
>> Why attempt to emulate these instructions at all, if we're not going
>> to handle a data access to emulated/special memory?
>> 
>> It seems that one of the following three cases must hold:
>> 
>> 1) The data accessed by the instruction is emulated/special memory.
>> 2) The instruction was fetched from emulated/special memory.
>> 3) The instruction has been modified since the VM-exit.
> 
> 4) The processor is in big real mode and you don't have unrestricted
> guest support in your processor (or you disabled EPT).

What about marking instructions that are not expected to access emulated
memory?

For now, we could WARN_ONCE if they do, which would pave a way to make
unrestricted guest mandatory.  Then we would drop instructions that were
not needed with the hope that they won't be.
(This would imply mandatory EPT.  Also a benefit, IMO.)

Westmere (the architecture to introduce unrestricted guest) is from
2010, which makes it close to being endangered by expired extended
warranties.