Messages in this thread | | | Date | Wed, 18 Oct 2023 20:03:14 +0800 | From | "Hou Wenlong" <> | Subject | Re: [PATCH 2/2] x86/sme: Mark the code as __head in mem_encrypt_identity.c |
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On Wed, Oct 18, 2023 at 06:20:15PM +0800, Ingo Molnar wrote: > > * Hou Wenlong <houwenlong.hwl@antgroup.com> wrote: > > > On Tue, Oct 17, 2023 at 08:52:46PM +0800, Ingo Molnar wrote: > > > > > > * Hou Wenlong <houwenlong.hwl@antgroup.com> wrote: > > > > > > > The functions sme_enable() and sme_encrypt_kernel() are only called by > > > > the head code which runs in identity virtual address. Therefore, it's > > > > better to mark them as __head as well. > > > > > > > > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> > > > > --- > > > > arch/x86/include/asm/mem_encrypt.h | 8 ++++---- > > > > arch/x86/mm/mem_encrypt_identity.c | 27 ++++++++++++++------------- > > > > 2 files changed, 18 insertions(+), 17 deletions(-) > > > > > > > > diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h > > > > index 359ada486fa9..48469e22a75e 100644 > > > > --- a/arch/x86/include/asm/mem_encrypt.h > > > > +++ b/arch/x86/include/asm/mem_encrypt.h > > > > @@ -46,8 +46,8 @@ void __init sme_unmap_bootdata(char *real_mode_data); > > > > > > > > void __init sme_early_init(void); > > > > > > > > -void __init sme_encrypt_kernel(struct boot_params *bp); > > > > -void __init sme_enable(struct boot_params *bp); > > > > +void sme_encrypt_kernel(struct boot_params *bp); > > > > +void sme_enable(struct boot_params *bp); > > > > > > > > int __init early_set_memory_decrypted(unsigned long vaddr, unsigned long size); > > > > int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size); > > > > @@ -75,8 +75,8 @@ static inline void __init sme_unmap_bootdata(char *real_mode_data) { } > > > > > > > > static inline void __init sme_early_init(void) { } > > > > > > > > -static inline void __init sme_encrypt_kernel(struct boot_params *bp) { } > > > > -static inline void __init sme_enable(struct boot_params *bp) { } > > > > +static inline void sme_encrypt_kernel(struct boot_params *bp) { } > > > > +static inline void sme_enable(struct boot_params *bp) { } > > > > > > So I think we should preserve the previous convention of marking functions > > > __init in the header-declaration and at the definition site as well, and do > > > the same with __head as well? > > > > > Hi Ingo, > > > > I tried to include <asm/init.h> into <asm/mem_encrypt.h> and mark the > > function declaration as __head, but it resulted in a build failure. This > > is because <asm/init.h> is not self-contained; the type "pgd_t" is > > defined in <asm/pgtable_types.h>, which includes <asm/mem_encrypt.h>, > > leading to mutual inclusion of header files. To avoid the issue of > > complicated header file inclusion, I removed the annotation from the > > function declaration. > > The right solution at that point is to make <asm/init.h> self-contained... >
The "pgd_t" is a typedef declaration in <asm/pgtable_types.h>, so it cannot be forward declared. Therefore, I had to include <asm/pgtable_types.h> into <asm/init.h> to make it self-contained. However, <asm/pgtable_types.h> includes <asm/mem_encrypt.h>. If I include <asm/init.h> into <asm/mem_encrypt.h> to mark functions as __head in the header-declaration, it would result in mutual inclusion of header files. It appears that <asm/mem_encrypt.h> is a base header that is included in multiple headers, so adding one more header to it would complicate things. In reality, if it is acceptable, I could move the __head definition into <asm/mem_encrypt.h>.
> > Actually, initially, I noticed that the __init definition is in > > <linux/init.h>, so I first placed the __head definition in > > <linux/init.h> as well. However, this conflicted with the local variable > > in the "list_next_or_null_rcu" macro in <linux/rculist.h>. Then I > > realized that __head was only used in x86, so I made the decision to put > > it in the architecture-specific header. Considering simplicity, I chose > > to put the definition in <asm/init.h>. I also attempted to put the > > definition in other headers such as <asm/boot.h> and > > <asm/bootparam_utils.h>, and included them in <asm/mem_encrypt.h>, but > > the build still failed. > > When exporting a localized definition you should consider namespace > collisions - the name '__head' is way too generic, no wonder it caused > problems elsewhere. > > I'd suggest naming it __init_head or so, but still keep it in a x86-only > header. > > I presume keeping it all in the separate section and widening its usage has a > specific purpose? Please outline that in the changelog as well. >
Based on my understanding, the __head section contains the early boot code that runs at a low identity address instead of the compile-time address. Therefore, it must use RIP-relative addressing to access memory. This makes the __head section special. However, when it comes to C source code, the compiler may generate absolute addressing, which can result in boot failure. That's why the fixup_pointer() function is introduced in head64.c. So maybe we could consider validating the memory access instructions in this section using objtool to ensure that the generated instructions are PC-relative. Then we should mark all the early boot code as __head.
Thanks!
> Ie. instead of mechanical patches that try to follow existing patterns > cargo-cult style, this area of x86 code requires well-argued, well thought > out patches that show background knowledge of the area. > > Thanks, > > Ingo
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