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

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Date:   Mon, 10 Sep 2018 14:27:27 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active

On Fri, Sep 07, 2018 at 12:57:30PM -0500, Brijesh Singh wrote:
> Currently, the per-cpu pvclock data is allocated dynamically when
> cpu > HVC_BOOT_ARRAY_SIZE.

Well no, you need to write this correctly - what is "cpu >
HVC_BOOT_ARRAY_SIZE" ?!

( I know what it is but I know it only because I've looked at that code before.  )

So no, please explain it in English not in code.

> 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.

This sentence is a good example about how to explain stuff in commit
messages.

> The C-bit works on a page,

"The C-bit determines the encryption status of a 4K page."

> hence we will be required to perform a

Use passive tone in your commit message: no "we", etc...

> 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

"... will waste *a* fairly sizeable amount of ..."

> a separate 4k allocation.

Start new paragraph here and use passive tone.

> Let's define a second array for the SEV case to
> statically allocate for NR_CPUS and put this array in .data..decrypted

NR_CPUS needs explaining for the unenlightened reader. Also,

	"... put this array in *the* .data..decrypted section... "

> section so that its mapped with C=0 during boot.

<---- newline here.

> 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

"... since *the* second array... "

s/hence //

> free it when encryption is not active.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: kvm@...r.kernel.org
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: kvm@...r.kernel.org
> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> ---
>  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);

Proper prefixing:

"mem_encrypt_free_decrypted"

or so

>  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;

Hmm, so worst case that's 64 4K pages:

(8192*32)/4096 = 64 4K pages.

Now, the real question from all this SNAFU is, why can't all those point
to a single struct pvclock_vsyscall_time_info and all CPUs read a single
thing? Why do they have to be per-CPU and thus waste so much memory?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--