From: Borislav Petkov <bp@alien8.de>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com,
pbonzini@redhat.com, tglx@linutronix.de, x86@kernel.org,
jgross@suse.com
Subject: Re: [RFC][PATCH 01/34] x86/xen: Explain why calling get_cpu_cap() so early is a hack
Date: Thu, 7 Mar 2024 17:27:52 +0100 [thread overview]
Message-ID: <20240307162752.GAZenrCDqs0lMTjmu1@fat_crate.local> (raw)
In-Reply-To: <20240222183928.0D8942B2@davehans-spike.ostc.intel.com>
On Thu, Feb 22, 2024 at 10:39:28AM -0800, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> xen_start_kernel() some of the first C code run "Xen PV" systems.
That sentence reads weird.
> It precedes normally very early things like setup_arch() or the
> processor initialization code.
>
> That means that 'boot_cpu_data' is garbage. It has not even
s/is/contains/
> established the utter basics like if the CPU supports the CPUID
> instruction. Unfortunately get_cpu_cap() requires this exact
> information.
>
> Nevertheless xen_start_kernel() calls get_cpu_cap(). But it
> works out in practice because it's looking for the NX bit which
> comes from an extended CPUID leaf that doesn't depend on
> c->cpuid_level being set. This also implicitly assumes that
> Xen PV guests support CPUID.
>
> Leave the hack in place, but at least explain some of what is
> going on.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Juergen Gross <jgross@suse.com>
> ---
>
> b/arch/x86/xen/enlighten_pv.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff -puN arch/x86/xen/enlighten_pv.c~xen-explain1 arch/x86/xen/enlighten_pv.c
> --- a/arch/x86/xen/enlighten_pv.c~xen-explain1 2024-02-22 10:08:48.404451146 -0800
> +++ b/arch/x86/xen/enlighten_pv.c 2024-02-22 10:08:48.404451146 -0800
> @@ -1372,7 +1372,11 @@ asmlinkage __visible void __init xen_sta
> /* Get mfn list */
> xen_build_dynamic_phys_to_machine();
>
> - /* Work out if we support NX */
> + /*
> + * This is a hack. 'boot_cpu_data' is not filled out enough
> + * for get_cpu_cap() to function fully. But it _can_ fill out
> + * the leaves where NX is. Gross, but it works.
> + */
> get_cpu_cap(&boot_cpu_data);
> x86_configure_nx();
If all it needs is to figure out whether it supports NX, just do that by
foot here and get rid of the get_cpu_cap(&boot_cpu_data) call
completely.
It won't be a hack anymore.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2024-03-07 16:28 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-22 18:39 [RFC][PATCH 00/34] [RFC] x86: Rework system-wide configuration masquerading as per-cpu data Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 01/34] x86/xen: Explain why calling get_cpu_cap() so early is a hack Dave Hansen
2024-03-07 16:27 ` Borislav Petkov [this message]
2024-02-22 18:39 ` [RFC][PATCH 02/34] x86/xen: Remove early "debug" physical address lookups Dave Hansen
2024-03-07 17:01 ` Borislav Petkov
2024-03-11 13:16 ` Jürgen Groß
2024-02-22 18:39 ` [RFC][PATCH 03/34] x86/pci: Assume that clflush size is always provided Dave Hansen
2024-03-08 8:57 ` Borislav Petkov
2024-02-22 18:39 ` [RFC][PATCH 04/34] x86/mm: Introduce physical address limit helper Dave Hansen
2024-02-27 11:05 ` Huang, Kai
2024-02-22 18:39 ` [RFC][PATCH 05/34] x86/cpu: Move /proc per-cpu ->x86_phys_bits reference to global one Dave Hansen
2024-02-27 11:05 ` Huang, Kai
2024-02-22 18:39 ` [RFC][PATCH 06/34] x86/boot: Use consistent value for iomem_resource.end Dave Hansen
2024-02-27 10:59 ` Huang, Kai
2024-02-28 14:22 ` Zhang, Rui
2024-02-22 18:39 ` [RFC][PATCH 07/34] x86/mm: Introduce virtual address space limit helper Dave Hansen
2024-02-27 11:09 ` Huang, Kai
2024-02-22 18:39 ` [RFC][PATCH 08/34] x86/cpu: Add CLFLUSH size helper Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 09/34] x86/cpu: Introduce address configuration structure Dave Hansen
2024-02-27 23:47 ` Huang, Kai
2024-02-28 17:29 ` Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 10/34] x86/cpu/amd: Use new "address configuration" infrastructure Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 11/34] x86/cpu/intel: Prepare MKTME for " Dave Hansen
2024-02-23 11:33 ` Kirill A. Shutemov
2024-02-23 16:22 ` Dave Hansen
2024-02-26 12:14 ` Kirill A. Shutemov
2024-02-27 21:48 ` Huang, Kai
2024-02-28 15:20 ` Kirill A. Shutemov
2024-02-28 16:57 ` Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 12/34] x86/cpu/intel: Actually use "address configuration" infrastructure for MKTME Dave Hansen
2024-02-23 11:41 ` Kirill A. Shutemov
2024-02-23 16:16 ` Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 13/34] x86/boot: Use address reduction config to handle erratum Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 14/34] x86/cpu: Remove default physical address space settings Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 15/34] x86/cpu: Remove default x86_phys_bits assignment Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 16/34] x86/cpu: Move physical address limit out of cpuinfo_x86 Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 17/34] x86/cpu: Move virtual " Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 18/34] x86/cpu/centaur: Move cache alignment override to BSP init Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 19/34] x86/cpu: Introduce cache alignment multiplier Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 20/34] x86/cpu: Remove superfluous cache alignment assignments Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 21/34] x86/cpu: Consolidate CLFLUSH size setting Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 22/34] x86/cpu: Move CLFLUSH size into global config Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 23/34] x86/cpu: Move cache alignment configuration to global struct Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 24/34] x86/cpu: Establish 'min_cache_bits' configuration Dave Hansen
2024-02-22 18:39 ` [RFC][PATCH 25/34] x86/cpu: Move cache bits to global config Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 26/34] x86/cpu: Zap superfluous get_cpu_address_sizes() Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 27/34] x86/cpu: Enforce read-only x86_config state (lightly) Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 28/34] x86/cpu: Return sane defaults for early x86_config reads Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 29/34] x86/xen: Remove extra get_cpu_address_sizes() call Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 30/34] x86/cpu/centaur: Mark BSP init function as __init Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 31/34] x86/cpu/intel: " Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 32/34] x86/cpu/amd: Move memory encryption detection Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 33/34] x86/cpu: Make get_cpu_address_sizes() static and __init Dave Hansen
2024-02-22 18:40 ` [RFC][PATCH 34/34] x86/cpu: Mark new boot CPU and config structures appropriately Dave Hansen
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