linux-kernel - Re: [PATCH v15 22/23] x86/mce: Improve error log of kernel space TDX #MC due to erratum

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Message-ID: <2394202d237b4a74440fba1a267652335b53b71d.camel@intel.com>
Date:   Tue, 5 Dec 2023 20:08:34 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "bp@...en8.de" <bp@...en8.de>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
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        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
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        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
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        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v15 22/23] x86/mce: Improve error log of kernel space TDX
 #MC due to erratum

On Tue, 2023-12-05 at 20:56 +0100, Borislav Petkov wrote:
> On Tue, Dec 05, 2023 at 07:41:41PM +0000, Huang, Kai wrote:
> > -static const char *mce_memory_info(struct mce *m)
> > +static const char *mce_dump_aux_info(struct mce *m)
> >  {
> > -       if (!m || !mce_is_memory_error(m) || !mce_usable_address(m))
> > -               return NULL;
> > -
> >         /*
> > -        * Certain initial generations of TDX-capable CPUs have an
> > -        * erratum.  A kernel non-temporal partial write to TDX private
> > -        * memory poisons that memory, and a subsequent read of that
> > -        * memory triggers #MC.
> > -        *
> > -        * However such #MC caused by software cannot be distinguished
> > -        * from the real hardware #MC.  Just print additional message
> > -        * to show such #MC may be result of the CPU erratum.
> > +        * Confidential computing platforms such as TDX platforms
> > +        * may occur MCE due to incorrect access to confidential
> > +        * memory.  Print additional information for such error.
> >          */
> > -       if (!boot_cpu_has_bug(X86_BUG_TDX_PW_MCE))
> > +       if (!m || !mce_is_memory_error(m) || !mce_usable_address(m))
> >                 return NULL;
> 
> What's the point of doing this on !TDX? None.

OK. I'll move this inside tdx_get_mce_info(). 

> 
> > -       return !tdx_is_private_mem(m->addr) ? NULL :
> > -               "TDX private memory error. Possible kernel bug.";
> > +       if (platform_tdx_enabled())
> 
> So is this the "host is TDX" check?
> 
> Not a X86_FEATURE flag but something homegrown. And Kirill is trying to
> switch the CC_ATTRs to X86_FEATURE_ flags for SEV but here you guys are
> using something homegrown.
> 
> why not a X86_FEATURE_ flag?
> 

The difference is for TDX host the kernel needs to initialize the TDX module
first before TDX can be used.  The module initialization is done at runtime, and
the platform_tdx_enabled() here only returns whether the BIOS has enabled TDX.

IIUC the X86_FEATURE_ flag doesn't suit this purpose because based on my
understanding the flag being present means the kernel has done some enabling
work and the feature is ready to use.