linux-kernel - Re: [PATCH 1/3] x86/cpufeatures: Add support for cpuid leaf 80000021/EAX (FeatureExt2Eax)

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Date:   Tue, 15 Nov 2022 17:10:50 -0600
From:   Kim Phillips <kim.phillips@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Babu Moger <Babu.Moger@....com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/cpufeatures: Add support for cpuid leaf
 80000021/EAX (FeatureExt2Eax)

On 11/4/22 4:48 PM, Borislav Petkov wrote:
> On Fri, Nov 04, 2022 at 04:36:49PM -0500, Kim Phillips wrote:
>> AMD Zen4 processors advertise features in this leaf.
>> Add the leaf and its Automatic IBRS feature bit.
>>
>> Note: New whole leaf (vs a bit) due to propagation via KVM
>> later in this series.
> 
> No, not a separate leaf - use scattered.c
>  > For an example what to do for KVM, see
> 
> https://lore.kernel.org/r/20221103025030.78371-1-jiaxi.chen@linux.intel.com

That adds features that are mutually exclusive between
kvm and the host kernel, unlike AUTOIBRS.

When trying to wire up a scattered host AUTOIBRS version up to
kvm, I couldn't get past all the reverse_cpuid_check()
BUILD_BUGs demanding exclusivity between h/w and "Linux"
(s/w) FEATUREs.

Is there an example of a scattered feature that gets both its
boot_cpu_has() and guest_cpuid_has() satisfied in the same build?

If not, I'll resubmit like this original submission - AUTOIBRS in
a separate h/w leaf.

Thanks,

Kim