linux-kernel - Re: [PATCH v7 11/13] KVM: x86: Implement Intel Processor Trace MSRs read/write

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Date:   Fri, 4 May 2018 23:47:41 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Luwei Kang <luwei.kang@...el.com>
Cc:     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 11/13] KVM: x86: Implement Intel Processor Trace MSRs
 read/write

On 04/05/2018 12:11, Alexander Shishkin wrote:
>> +	 */
>> +	if ((data & RTIT_CTL_TRACEEN) && !(data & RTIT_CTL_TOPA) &&
>> +		!(data & RTIT_CTL_FABRIC_EN) &&
>> +		!__pt_cap_get(vmx->pt_desc.caps, PT_CAP_single_range_output))
> You seem to be doing a lot of __pt_cap_get()s on each wrmsr. Did you consider
> decoding the capabilities once and storing the decoded values instead, so
> that in functions like these you can access them by
> 
>    if (vmx->pt_desc.caps[PT_CAP_single_range_output]) ...
> 
> ?

Or pt_cap_get could use a switch statement in pt_cap_get and make it
__always_inline.  The argument is always a constant except for pt_cap_show.

What you say is also a possibility though.  Are you okay with adding a
PT_CAP_first_unused at the end of enum pt_capabilities, so that the
array can be sized?

Thanks,

Paolo