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

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Date:   Fri, 13 Oct 2023 16:03:02 -0700
From:   "Compostella, Jeremy" <jeremy.compostella@...el.com>
To:     kirill.shutemov@...ux.intel.com, "Huang, Kai" <kai.huang@...el.com>
Cc:     "mingo@...nel.org" <mingo@...nel.org>,
        "Li, Xin3" <xin3.li@...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

"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com> writes:
> 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?

No override indeed as get_cpu_address_sizes() is always called before
early_init_intel or init_intel().

- init/main.c::start_kernel()
  - arch/x86/kernel/setup.c::setup_arch()
    - arch/x86/kernel/cpu/common.c::early_cpu_init()
      - early_identify_cpu()
        - get_cpu_address_sizes(c)
          c->x86_phys_bits = eax & 0xff;
        - arch/x86/kernel/cpu/intel.c::early_init_intel()
          - detect_tme()
            c->x86_phys_bits -= keyid_bits;
  - arch/x86/kernel/cpu/common.c::arch_cpu_finalize_init()
    - identify_boot_cpu()
      - identify_cpu()
        - get_cpu_address_sizes(c)
          c->x86_phys_bits = eax & 0xff;
        - arch/x86/kernel/cpu/intel.c::init_intel()
          - early_init_intel()
            - detect_tme()
              c->x86_phys_bits -= keyid_bits;

> 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.

This change is not modifying the CPU indent code, this is just
re-ordering detect_tme() call in the intel specifics hook so that the
information is available earlier as it is needed by
generic_get_mtrr(). This is similar to what is done in
arch/x86/kernel/cpu/amd.c.