Re: Should KVM_GUEST stop depending on PARAVIRT?

[Arjan van de Ven <arjan@linux.intel.com>]

On 7/27/2015 11:45 AM, Andy Lutomirski wrote:

>> With PARAVIRT=y it never #GPs:
>>
>>          .read_msr = native_read_msr_safe,
>>          .write_msr = native_write_msr_safe,
>>
>> I don't remember if it's this way on bare-metal too.
>
> Oh, whoops, I missed the "_safe".  IMO that's just a bug, and I guess
> KVM relies on it?

btw it's a common misperception that _safe is actually safe;-)
they're still highly dangerous, just they won't fault on the linux side


>
> ISTM the host should be fixed so that a non-PARAVIRT guest won't crash
> when using perf (if it indeed currently crashes) and/or the perf code
> should be fixed to work without this bug^Wfeature.
>
> Then KVM_GUEST kernels could be de-bloated by dropping PARAVIRT.
>
> Hi Arjan- A quick and dirty measurement suggests that this would save
> 2-3 ms when booting a KVM_GUEST=y kernel under KVM by turning
> apply_paravirt into a noop.

nice