linux-kernel - Re: [PATCH v3 3/9] KVM: x86: Add Intel Processor Trace virtualization mode

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Message-ID: <9a15e683-8627-0fc6-a95d-3ac6a0468509@redhat.com>
Date:   Thu, 30 Nov 2017 13:25:34 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Luwei Kang <luwei.kang@...el.com>, kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
        joro@...tes.org, Chao Peng <chao.p.peng@...ux.intel.com>
Subject: Re: [PATCH v3 3/9] KVM: x86: Add Intel Processor Trace virtualization
 mode

On 27/11/2017 21:23, Luwei Kang wrote:
> +/* Default is host guest mode. */
> +static int __read_mostly pt_mode = PT_MODE_HOST_GUEST;
> +module_param(pt_mode, int, S_IRUGO);
> +

This will cause a slowdown on new processors.  Please make the default
PT_SYSTEM until that is addressed (with a combination of XSAVES, caching
the host rtit_ctl at the kernel level, or something like that).

Paolo