Re: [PATCH v4 1/3] x86/sev: Add new dump_rmp parameter to snp_leak_pages() API

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 10, 2025, Ashish Kalra wrote:
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 00475b814ac4..7a1ae990b15f 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -635,10 +635,15 @@ void snp_dump_hva_rmpentry(unsigned long address);
>  int psmash(u64 pfn);
>  int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, u32 asid, bool immutable);
>  int rmp_make_shared(u64 pfn, enum pg_level level);
> -void snp_leak_pages(u64 pfn, unsigned int npages);
> +void __snp_leak_pages(u64 pfn, unsigned int npages, bool dump_rmp);
>  void kdump_sev_callback(void);
>  void snp_fixup_e820_tables(void);
>  
> +static inline void snp_leak_pages(u64 pfn, unsigned int pages)
> +{
> +	__snp_leak_pages(pfn, pages, true);
> +}
> +
>  static inline void sev_evict_cache(void *va, int npages)
>  {
>  	volatile u8 val __always_unused;
> @@ -668,6 +673,7 @@ static inline int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, u32 as
>  	return -ENODEV;
>  }
>  static inline int rmp_make_shared(u64 pfn, enum pg_level level) { return -ENODEV; }
> +static inline void __snp_leak_pages(u64 pfn, unsigned int npages, bool dump_rmp) {}

This stub is unnecessary.  As pointed out elsewhere[*], I'm pretty sure all these
stubs are unnecessary.

Oof.  Even worse, the stubs appear to be actively hiding bugs.  The APIs are
guarded with CONFIG_KVM_AMD_SEV=y, but **KVM** doesn't call any of these outside
of SEV code.  I.e. if *KVM* were the only user, the stubs would just be dead code.

But the below build failures show that they aren't dead code, which means that
kernels with CONFIG_KVM_AMD_SEV=n will silently (until something explodes) do the
wrong thing, because the stubs are hiding the missing dependencies.

arch/x86/boot/startup/sev-shared.c: In function ‘pvalidate_4k_page’:
arch/x86/boot/startup/sev-shared.c:820:17: error: implicit declaration of function ‘sev_evict_cache’ [-Wimplicit-function-declaration]
  820 |                 sev_evict_cache((void *)vaddr, 1);
      |                 ^~~~~~~~~~~~~~~
  AR      arch/x86/realmode/built-in.a
arch/x86/coco/sev/core.c: In function ‘pvalidate_pages’:
arch/x86/coco/sev/core.c:386:25: error: implicit declaration of function ‘sev_evict_cache’ [-Wimplicit-function-declaration]
  386 |                         sev_evict_cache(pfn_to_kaddr(e->gfn), e->pagesize ? 512 : 1);
      |                         ^~~~~~~~~~~~~~~
arch/x86/mm/mem_encrypt.c: In function ‘mem_encrypt_setup_arch’:
arch/x86/mm/mem_encrypt.c:112:17: error: implicit declaration of function ‘snp_fixup_e820_tables’ [-Wimplicit-function-declaration]
  112 |                 snp_fixup_e820_tables();
      |                 ^~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/fault.c: In function ‘show_fault_oops’:
arch/x86/mm/fault.c:587:17: error: implicit declaration of function ‘snp_dump_hva_rmpentry’ [-Wimplicit-function-declaration]
  587 |                 snp_dump_hva_rmpentry(address);
      |                 ^~~~~~~~~~~~~~~~~~~~~
arch/x86/kernel/cpu/amd.c: In function ‘bsp_determine_snp’:
arch/x86/kernel/cpu/amd.c:370:21: error: implicit declaration of function ‘snp_probe_rmptable_info’ [-Wimplicit-function-declaration]
  370 |                     snp_probe_rmptable_info()) {
      |                     ^~~~~~~~~~~~~~~~~~~~~~~
  AR      drivers/iommu/amd/built-in.a
  AR      drivers/iommu/built-in.a
  AR      drivers/built-in.a

[*] https://lore.kernel.org/all/aMHP5EO-ucJGdHXz@google.com