Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

[Paolo Bonzini]

On 07/09/2018 19:57, Brijesh Singh wrote:
> Currently, the per-cpu pvclock data is allocated dynamically when
> cpu > HVC_BOOT_ARRAY_SIZE. The physical address of this variable is
> shared between the guest and the hypervisor hence it must be mapped as
> unencrypted (ie. C=0) when SEV is active.
> 
> The C-bit works on a page, hence we will be required to perform a
> full 4k page allocation to store a single 32-byte pvclock variable. It
> will waste fairly sizeable amount of memory since each CPU will be doing
> a separate 4k allocation. Let's define a second array for the SEV case to
> statically allocate for NR_CPUS and put this array in .data..decrypted
> section so that its mapped with C=0 during boot. The .data..decrypted
> section has a big chunk of memory that is currently unused. And since
> second array will be used only when memory encryption is active hence
> free it when encryption is not active.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/mem_encrypt.h |  4 ++++
>  arch/x86/kernel/kvmclock.c         | 14 ++++++++++++++
>  arch/x86/kernel/vmlinux.lds.S      |  3 +++
>  arch/x86/mm/init.c                 |  3 +++
>  arch/x86/mm/mem_encrypt.c          | 10 ++++++++++
>  5 files changed, 34 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> index 802b2eb..cc46584 100644
> --- a/arch/x86/include/asm/mem_encrypt.h
> +++ b/arch/x86/include/asm/mem_encrypt.h
> @@ -48,11 +48,13 @@ int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size);
>  
>  /* Architecture __weak replacement functions */
>  void __init mem_encrypt_init(void);
> +void __init free_decrypted_mem(void);
>  
>  bool sme_active(void);
>  bool sev_active(void);
>  
>  #define __decrypted __attribute__((__section__(".data..decrypted")))
> +#define __decrypted_aux __attribute__((__section__(".data..decrypted.aux")))
>  
>  #else	/* !CONFIG_AMD_MEM_ENCRYPT */
>  
> @@ -80,6 +82,7 @@ static inline int __init
>  early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0; }
>  
>  #define __decrypted
> +#define __decrypted_aux
>  
>  #endif	/* CONFIG_AMD_MEM_ENCRYPT */
>  
> @@ -93,6 +96,7 @@ early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0;
>  #define __sme_pa_nodebug(x)	(__pa_nodebug(x) | sme_me_mask)
>  
>  extern char __start_data_decrypted[], __end_data_decrypted[];
> +extern char __start_data_decrypted_aux[];
>  
>  #endif	/* __ASSEMBLY__ */
>  
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 376fd3a..6086b56 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -65,6 +65,15 @@ static struct pvclock_vsyscall_time_info
>  static struct pvclock_wall_clock wall_clock __decrypted;
>  static DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu);
>  
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +/*
> + * The auxiliary array will be used when SEV is active. In non-SEV case,
> + * it will be freed by free_decrypted_mem().
> + */
> +static struct pvclock_vsyscall_time_info
> +			hv_clock_aux[NR_CPUS] __decrypted_aux;
> +#endif
> +
>  static inline struct pvclock_vcpu_time_info *this_cpu_pvti(void)
>  {
>  	return &this_cpu_read(hv_clock_per_cpu)->pvti;
> @@ -269,6 +278,11 @@ static int kvmclock_setup_percpu(unsigned int cpu)
>  	/* Use the static page for the first CPUs, allocate otherwise */
>  	if (cpu < HVC_BOOT_ARRAY_SIZE)
>  		p = &hv_clock_boot[cpu];
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	/* Use the static page from auxiliary array instead of allocating it. */
> +	else if (sev_active())
> +		p = &hv_clock_aux[cpu - HVC_BOOT_ARRAY_SIZE];
> +#endif
>  	else
>  		p = kzalloc(sizeof(*p), GFP_KERNEL);
>  
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 4cb1064..bde287a 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -77,6 +77,9 @@ jiffies_64 = jiffies;
>  	. = ALIGN(PMD_SIZE);					\
>  	__start_data_decrypted = .;				\
>  	*(.data..decrypted);					\
> +	. = ALIGN(PAGE_SIZE);					\
> +	__start_data_decrypted_aux = .;				\
> +	*(.data..decrypted.aux);				\
>  	. = ALIGN(PMD_SIZE);					\
>  	__end_data_decrypted = .;				\
>  
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 7a8fc26..052b279 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -815,9 +815,12 @@ void free_kernel_image_pages(void *begin, void *end)
>  		set_memory_np_noalias(begin_ul, len_pages);
>  }
>  
> +void __weak free_decrypted_mem(void) { }
> +
>  void __ref free_initmem(void)
>  {
>  	e820__reallocate_tables();
> +	free_decrypted_mem();
>  
>  	free_kernel_image_pages(&__init_begin, &__init_end);
>  }
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index b2de398..9a08c52 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -348,6 +348,16 @@ bool sev_active(void)
>  EXPORT_SYMBOL(sev_active);
>  
>  /* Architecture __weak replacement functions */
> +void __init free_decrypted_mem(void)
> +{
> +	if (mem_encrypt_active())
> +		return;
> +
> +	free_init_pages("unused decrypted",
> +			(unsigned long)__start_data_decrypted_aux,
> +			(unsigned long)__end_data_decrypted);
> +}
> +
>  void __init mem_encrypt_init(void)
>  {
>  	if (!sme_me_mask)
> 

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>