Re: [PATCH v13 08/12] KVM: x86: Add Intel PT context switch for each vcpu

[Paolo Bonzini]

On 24/10/2018 12:13, Alexander Shishkin wrote:
> Luwei Kang <luwei.kang@xxxxxxxxx> writes:
> 
>> +static void pt_guest_enter(struct vcpu_vmx *vmx)
>> +{
>> +	if (pt_mode == PT_MODE_SYSTEM)
>> +		return;
>> +
>> +	/* Save host state before VM entry */
>> +	rdmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
>> +
>> +	/*
>> +	 * Set guest state of MSR_IA32_RTIT_CTL MSR (PT will be disabled
>> +	 * on VM entry when it has been disabled in guest before).
>> +	 */
>> +	vmcs_write64(GUEST_IA32_RTIT_CTL, vmx->pt_desc.guest.ctl);
>> +
>> +	if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
>> +		wrmsrl(MSR_IA32_RTIT_CTL, 0);
>> +		pt_save_msr(&vmx->pt_desc.host, vmx->pt_desc.addr_range);
>> +		pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.addr_range);
>> +	}
>> +}
> 
> From my side this is still a NAK, because [1].
> 
> [1] https://marc.info/?l=kvm&m=153847567226248&w=2

Then you should have replied to
https://marc.info/?l=kvm&m=153865386015249&w=2 instead of having Luwei
do the work for nothing.

Quoting from there:

>> One shouldn't have to enable or disable anything in KVM to stop it from
>> breaking one's existing workflow. That makes no sense.
> 
> If you "have to enable or disable anything" it means you have to
> override the default.  But the default in this patches is "no change
> compared to before the patches", leaving tracing of both host and guest
> entirely to the host, so I don't understand your remark.  What workflow
> is broken?
> 
>> There already are controls in perf that enable/disable guest tracing.
> 
> You are confusing "tracing guest from the host" and "the guest can trace
> itself".  This patchset is adding support for the latter, and that
> affects directly whether the tracing CPUID leaf can be added to the
> guest.  Therefore it's not perf that can decide whether to turn it on;
> KVM must know it when /dev/kvm is opened, which is why it is a module
> parameter.

I'd be happier if we found an agreement, but without discussion that
just won't happen.

Also, is there an existing interface to write a record into a tracing
buffer?

Paolo