linux-kernel - Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature

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Message-ID: <20211006050309.ldn5myznmpurnomm@treble>
Date:   Tue, 5 Oct 2021 22:03:09 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature

On Tue, Oct 05, 2021 at 09:33:35PM -0700, Kuppuswamy, Sathyanarayanan wrote:
> 
> 
> On 10/5/21 8:42 PM, Josh Poimboeuf wrote:
> > > is_tdx_guest was mainly introduced to support cc_platform_has()
> > > API in early boot calls (similar to sme_me_mask in AMD code).
> > > Regarding FEATURE flag it will be useful for userspace tools to
> > > check the TDX feature support.
> > FEATURE flags can also be checked in the kernel, with boot_cpu_has().
> > Or am I missing something?
> 
> Yes, previously we have been using x86_feature_enabled() check in
> cc_platform_has() call. Now with the introduction of is_tdx_guest
> global variable, we don't use it in kernel. But I still want to
> keep the feature flag for user space use case.

I'm not suggesting getting rid of the feature flag.  I'm suggesting
getting rid of the global variable.  Is there a reason you can't check
the feature flag instead of checking the global variable?

-- 
Josh