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

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Message-ID: <20231003070659.hsjvnoc53agvms6c@box.shutemov.name>
Date:   Tue, 3 Oct 2023 10:06:59 +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 Tue, Oct 03, 2023 at 02:06:52AM +0000, Huang, Kai wrote:
> On Tue, 2023-10-03 at 01:47 +0300, kirill.shutemov@...ux.intel.com wrote:
> > 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.
> 
> After this patch, early_identify_cpu() calls get_cpu_address_sizes() first and
> then calls c_early_init(), which calls detect_tme().
> 
> So looks no override.  No?

We identify CPU twice: once via early_cpu_init() and the second time via
identify_boot_cpu()/identify_secondary_cpu(). I am talking about
early_cpu_init() codepath.

It might not matter in practice as of now, because it will get straight
later, but CPU ident code is mess as it is. Let's not make it even worse.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov