linux-kernel - Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode

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Date:   Thu, 3 May 2018 15:09:28 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Luwei Kang <luwei.kang@...el.com>, kvm@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
        joro@...tes.org, peterz@...radead.org, chao.p.peng@...ux.intel.com
Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace
 virtualization mode

On Thu, May 03, 2018 at 01:52:16PM +0200, Paolo Bonzini wrote:
> On 03/05/2018 13:32, Alexander Shishkin wrote:
> >> +static inline bool cpu_has_vmx_pt_use_gpa(void)
> >> +{
> >> +	return vmcs_config.cpu_based_2nd_exec_ctrl & SECONDARY_EXEC_PT_USE_GPA;
> >> +}
> > 
> > I can deduce the meaning of the previous one, but not this one, and there's
> > no explanation.
> 
> Within KVM, GPA always means guest physical address.

I see, thanks.

> >> +	if (pt_mode == PT_MODE_SYSTEM)
> >> +		vmexit_control &= ~(VM_EXIT_CLEAR_IA32_RTIT_CTL |
> >> +				    VM_EXIT_PT_CONCEAL_PIP);
> > 
> > Ok, so what we really want to know is: is there an encompassing PT
> > event on this cpu when we go into VMLAUNCH/VMRESTORE, right?
> > We can find this out from the pt_ctx and avoid the pt_mode entirely.
> > IOW, instead of having the 3 modes that you describe at the top, you
> > can use something like the following:
> > 
> > 1. Do we have an event in pt_ctx?
> >  * No -> Set up the context for VMX.
> >  * Yes -> 2. Is attr.exclude_guest set?
> >              * No -> Guest trace goes to the host's buffer, do nothing.
> > 	     * Yes -> Set up/switch the context for VMX.
> 
> Can you explain this more clearly?

Let's see; in the intel_pt driver we have a per-cpu PT "context", from which
we can tell if there is a host event that wants to trace the guest. This
should provide enough information to make a decision whether we want to
context switch PT MSRs or not on the spot, instead of having a module
parameter.

Regards,
--
Alex