linux-kernel - Re: [PATCH v3 1/2] x86/cpu/intel: Fix MTRR verification for TME enabled platforms

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Date:   Tue, 3 Oct 2023 01:47:52 +0300
From:   "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "mingo@...nel.org" <mingo@...nel.org>,
        "Li, Xin3" <xin3.li@...el.com>,
        "Compostella, Jeremy" <jeremy.compostella@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH v3 1/2] x86/cpu/intel: Fix MTRR verification for TME
 enabled platforms

On Fri, Sep 29, 2023 at 09:14:00AM +0000, Huang, Kai wrote:
> On Thu, 2023-09-28 at 15:30 -0700, Compostella, Jeremy wrote:
> > On TME enabled platform, BIOS publishes MTRR taking into account Total
> > Memory Encryption (TME) reserved bits.
> > 
> > generic_get_mtrr() performs a sanity check of the MTRRs relying on the
> > `phys_hi_rsvd' variable which is set using the cpuinfo_x86 structure
> > `x86_phys_bits' field.  But at the time the generic_get_mtrr()
> > function is ran the `x86_phys_bits' has not been updated by
> > detect_tme() when TME is enabled.
> > 
> > Since the x86_phys_bits does not reflect yet the real maximal physical
> > address size yet generic_get_mtrr() complains by logging the following
> > messages.
> > 
> >     mtrr: your BIOS has configured an incorrect mask, fixing it.
> >     mtrr: your BIOS has configured an incorrect mask, fixing it.
> >     [...]
> > 
> > In such a situation, generic_get_mtrr() returns an incorrect size but
> > no side effect were observed during our testing.
> > 
> > For `x86_phys_bits' to be updated before generic_get_mtrr() runs,
> > move the detect_tme() call from init_intel() to early_init_intel().
> 
> Hi,
> 
> This move looks good to me, but +Kirill who is the author of detect_tme() for
> further comments.
> 
> Also I am not sure whether it's worth to consider to move this to
> get_cpu_address_sizes(), which calculates the virtual/physical address sizes. 
> Thus it seems anything that can impact physical address size could be put there.

Actually, I am not sure how this patch works. AFAICS after the patch we
have the following callchain:

early_identify_cpu()
  this_cpu->c_early_init() (which is early_init_init())
    detect_tme()
      c->x86_phys_bits -= keyid_bits;
  get_cpu_address_sizes(c);
    c->x86_phys_bits = eax & 0xff;

Looks like get_cpu_address_sizes() would override what detect_tme() does.

I guess we reach the same detect_tme() again via c->c_init() (aka
init_intel()) codepath and get the value right again.

But it seems accidental.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov