linux-kernel - Re: [RFC PATCH] x86/fpu/xstate: Add more diagnostic information on inconsistent xstate sizes

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Date:   Wed, 12 Apr 2023 07:35:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Chintan M Patel <chintan.m.patel@...el.com>,
        Thiago Macieira <thiago.macieira@...el.com>
Subject: Re: [RFC PATCH] x86/fpu/xstate: Add more diagnostic information on
 inconsistent xstate sizes

On 4/11/23 18:21, Fenghua Yu wrote:
> In other words, splitting max_features into XCR0 and IA32_XSS and
> showing them individually provide more useful debug info than one single
> max_features value.
> 
> Does it make sense?

Not to me.

>> I still expect some acknowledgment of what is coded here for the
>> kernel calculation details.
> 
> The kernel calculation is shown in
> +        print_xstate_offset_size();
> +        pr_info("x86/fpu: total size: %u bytes\n", size);
> 
> Isn't that detailed enough to show offset and size of each xstate and
> sum of sizes?
> 
> After that,
> +    pr_info("x86/fpu: kernel_size from CPUID.0xd.0x%x:EBX: %u bytes\n",
> +               compacted ? 1 : 0, kernel_size);
> shows how kernel_size is calculated from CPUID?
> 
> Using the above debug info, a real platform CPUID issue is shown clearly.
> 
> What other details are needed?

I was kinda hoping this would be a simple, non-controversial patch that
would get us better debugging info the next time that the microcode or a
bad VMM screws up.  This patch isn't turning out to be as simple as I hoped.

I was wrong.  Let's just drop this.