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 02:06:52 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.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, 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?

> 
> 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.
> 

After this patch:

identify_cpu()	->
	generic_identify() ->
		get_cpu_address_sizes()		<----- (1)
	this_cpu->c_init() ->
		early_init_intel() ->
			detect_tme()		<----- (2)

(1) resets x86_phys_bits [*], but (2) is called rather soon so nothing wrong is
done between them for now.

But there's a window between (1) and (2) (and similarly in early_identify_cpu()
too).  Things can get wrong if people are careless, thus I said _perhaps_ it's
worth to consider to move detect_tme() to get_cpu_address_sizes().  But I don't
know.

[*]: on the other hand, (1) is necessary to make (2) right.