linux-kernel - Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism to read MSR

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <7899fcd9-3492-49d3-8097-a3ddefaaeef0@zytor.com>
Date: Wed, 23 Apr 2025 02:03:41 -0700
From: Xin Li <xin@...or.com>
To: Jürgen Groß <jgross@...e.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-hyperv@...r.kernel.org,
        virtualization@...ts.linux.dev, linux-pm@...r.kernel.org,
        linux-edac@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org, linux-hwmon@...r.kernel.org,
        netdev@...r.kernel.org, platform-driver-x86@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        acme@...nel.org, andrew.cooper3@...rix.com, peterz@...radead.org,
        namhyung@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        irogers@...gle.com, adrian.hunter@...el.com, kan.liang@...ux.intel.com,
        wei.liu@...nel.org, ajay.kaher@...adcom.com,
        bcm-kernel-feedback-list@...adcom.com, tony.luck@...el.com,
        pbonzini@...hat.com, vkuznets@...hat.com, seanjc@...gle.com,
        luto@...nel.org, boris.ostrovsky@...cle.com, kys@...rosoft.com,
        haiyangz@...rosoft.com, decui@...rosoft.com
Subject: Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism
 to read MSR

On 4/22/2025 4:12 AM, Jürgen Groß wrote:
>> +
>> +static __always_inline bool __rdmsrq(u32 msr, u64 *val, int type)
>> +{
>> +    bool ret;
>> +
>> +#ifdef CONFIG_XEN_PV
>> +    if (cpu_feature_enabled(X86_FEATURE_XENPV))
>> +        return __xenpv_rdmsrq(msr, val, type);
> 
> I don't think this will work for the Xen PV case.

Well, I have been testing the code on xen-4.17 coming with Ubuntu
24.04.2 LTS :)

> 
> X86_FEATURE_XENPV is set only after the first MSR is being read.

No matter whether the code works or not, good catch!

> 
> This can be fixed by setting the feature earlier, but it shows that the
> paravirt feature has its benefits in such cases.

See my other reply to let Xen handle all the details.

Plus the code actually works, I would actually argue the opposite :-P