Re: [PATCH v1 1/3] i386: Remove the limitation of IP payloads for Intel PT

[Paolo Bonzini]

On 28/09/20 07:19, Kang, Luwei wrote:
>>>> No, it's not possible.  KVM doesn't have a say on what the processor
>>>> writes in the tracing packets.
>>> Can KVM refuse to enable packet generation if CSbase is not zero and
>>> CPUID.(EAX=14H,ECX=0)[bit 31] seen by guest is different from host?
>>
>> Yes, but the processor could change operating mode (and hence CSbase) while
>> tracing is active.  This is very unlikely, since it would require nonzero 
>> CS-base
>> and a 32-bit host, but in principle not impossible (could be a firmware 
>> call, for
>> example).
>>
>> The only solution is for KVM to accept both, and for QEMU to refuse a setting
>> that does not match the host.
>>
> 
> So I need to add a patch in KVM to disabled the Intel PT when the
> CSbase is not zero and the guest LIP different from the host. And this
> limitation in qemu (disabled the PT when LIP is enabled in the host) can
> be remove. Is that right?

No, if a feature cannot be emulated, that means it cannot be enabled
unless it matches the host.  That's generally not a problem since Intel
PT is usually used only with "-cpu host".

Paolo