linux-kernel - Re: [RFC][PATCH 02/34] x86/xen: Remove early "debug" physical address lookups

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Date: Thu, 7 Mar 2024 18:01:39 +0100
From: Borislav Petkov <bp@...en8.de>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
	pbonzini@...hat.com, tglx@...utronix.de, x86@...nel.org,
	jgross@...e.com
Subject: Re: [RFC][PATCH 02/34] x86/xen: Remove early "debug" physical
 address lookups

On Thu, Feb 22, 2024 at 10:39:29AM -0800, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The __pa() facility is subject to debugging checks if CONFIG_DEBUG_VIRTUAL=y.
> One of those debugging checks is whether the physical address is valid
> on the platform.  That information is normally available via CPUID.  But
> the __pa() code currently looks it up in 'boot_cpu_data' which is not
> fully set up in early Xen PV boot.
> 
> The Xen PV code currently tries to get this info with
> get_cpu_address_sizes() which also depends on 'boot_cpu_data' to be at
> least somewhat set up.  The result is that the c->x86_phys_bits gets a
> sane value, but not one that has anything to do with the hardware.  In
> other words, the CONFIG_DEBUG_VIRTUAL checks are performed with what
> amounts to garbage inputs.
> 
> Garbage checks are worse than no check at all.  Move over to the
> "nodebug" variant to axe the checks.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Juergen Gross <jgross@...e.com>
> ---
> 
>  b/arch/x86/xen/enlighten_pv.c |    2 +-
>  b/arch/x86/xen/mmu_pv.c       |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Borislav Petkov (AMD) <bp@...en8.de>

At least as far as I can follow the Xen boot flow, yap, this makes
sense.

But let's have Jürgen confirm first.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette