linux-kernel - Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

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Message-ID: <b894ba89-27b2-88a9-6adf-7d53e2c51c02@citrix.com>
Date:   Thu, 14 Sep 2023 14:01:35 +0100
From:   andrew.cooper3@...rix.com
To:     Juergen Gross <jgross@...e.com>, Xin Li <xin3.li@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, jiangshanlai@...il.com
Subject: Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

On 14/09/2023 7:02 am, Juergen Gross wrote:
> On 14.09.23 06:47, Xin Li wrote:
>> Add an always inline API __wrmsrns() to embed the WRMSRNS instruction
>> into the code.
>>
>> Tested-by: Shan Kang <shan.kang@...el.com>
>> Signed-off-by: Xin Li <xin3.li@...el.com>
>
> In order to avoid having to add paravirt support for WRMSRNS I think
> xen_init_capabilities() should gain:
>
> +    setup_clear_cpu_cap(X86_FEATURE_WRMSRNS);

Xen PV guests will never ever see WRMSRNS.  Operating in CPL3, they have
no possible way of adjusting an MSR which isn't serialising, because
even the hypercall forms are serialising.

Xen only exposes the bit for HVM guests.

~Andrew