linux-kernel - Re: [RFC][PATCH 06/34] x86/boot: Use consistent value for iomem

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Date: Wed, 28 Feb 2024 14:22:26 +0000
From: "Zhang, Rui" <rui.zhang@...el.com>
To: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Huang,
 Kai" <kai.huang@...el.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>
CC: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de"
	<bp@...en8.de>
Subject: Re: [RFC][PATCH 06/34] x86/boot: Use consistent value for
 iomem_resource.end

On Tue, 2024-02-27 at 10:59 +0000, Huang, Kai wrote:
> On Thu, 2024-02-22 at 10:39 -0800, Dave Hansen wrote:
> > From: Dave Hansen <dave.hansen@...ux.intel.com>
> > 
> > The 'struct cpuinfo_x86' values (including 'boot_cpu_info') get
> > written and overwritten rather randomly.  They are not stable
> > during early boot and readers end up getting a random mishmash
> > of hard-coded defaults or CPUID-provided values based on when
> > the values are read.
> > 
> > iomem_resource.end is one of these users.  Because of where it
> > is called, it ended up seeing .x86_phys_bits==MAX_PHYSMEM_BITS
> > which is (mostly) a compile-time default.  But
> > iomem_resource.end is never updated if the runtime CPUID
> > x86_phys_bits is lower.
> > 
> > Set iomem_resource.end to the compile-time value explicitly.
> > It does not need to be precise as this is mostly to ensure
> > that insane values can't be reserved in 'iomem_resource'.
> > 
> > Make MAX_PHYSMEM_BITS available outside of sparsemem
> > configurations by removing the #ifdef CONFIG_SPARSEMEM in the
> > header.
> > 
> > Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> > ---
> > 
> >  b/arch/x86/include/asm/sparsemem.h |    3 ---
> >  b/arch/x86/kernel/setup.c          |   10 +++++++++-
> >  2 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff -puN arch/x86/kernel/setup.c~iomem_resource_end
> > arch/x86/kernel/setup.c
> > --- a/arch/x86/kernel/setup.c~iomem_resource_end        2024-02-22
> > 10:08:51.048554948 -0800
> > +++ b/arch/x86/kernel/setup.c   2024-02-22 10:21:04.485531464 -0800
> > @@ -51,6 +51,7 @@
> >  #include <asm/pci-direct.h>
> >  #include <asm/prom.h>
> >  #include <asm/proto.h>
> > +#include <asm/sparsemem.h>
> >  #include <asm/thermal.h>
> >  #include <asm/unwind.h>
> >  #include <asm/vsyscall.h>
> > @@ -813,7 +814,14 @@ void __init setup_arch(char **cmdline_p)
> >          */
> >         early_reserve_memory();
> >  
> > -       iomem_resource.end = (1ULL << x86_phys_bits()) - 1;
> > +       /*
> > +        * This was too big before.  It ended up getting
> > MAX_PHYSMEM_BITS
> > +        * even if .x86_phys_bits was eventually lowered below
> > that.
> > +        * But that was evidently harmless, so leave it too big,
> > but
> > +        * set it explicitly to MAX_PHYSMEM_BITS instead of taking
> > a
> > +        * trip through .x86_phys_bits.
> > +        */
> > +       iomem_resource.end = (1ULL << MAX_PHYSMEM_BITS) - 1;
> 
> Paolo's patchset to move MKTME keyid bits detection to
> early_cpu_init() was
> merged to tip:x86/urgent, so looks it will land to Linus's tree
> before this
> series:
> 
> https://lore.kernel.org/lkml/eff34df2-fdc1-4ee0-bb8d-90da386b7cb6@intel.com/T/
> 
> Paplo's series actually moves the reduction of x86_phys_bits before
> setting the
> iomem_resource.end here, so after rebasing the changelog/comment
> seems don't
> apply anymore.

My understanding is that the below order is always true,
setup_arch()
	early_cpu_init()
		get_cpu_address_sizes()
	iomem_resource.end = (1ULL << x86_phys_bits()) - 1;
with or without the above patch.

> 
> Perhaps we can get rid of this patch and just set iomem_resource.end
> based on
> x86_phys_bits()?
> 
Agreed.

thanks,
rui