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Message-ID: <20180906140738.GA370@linux.intel.com>
Date: Thu, 6 Sep 2018 07:07:39 -0700
From: Sean Christopherson <sean.j.christopherson@...el.com>
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>,
Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
when SEV is active
On Thu, Sep 06, 2018 at 06:43:02AM -0500, 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.
>
> When SEV is active, we will be wasting fairly sizeable amount of memory
> since each CPU will be doing a separate 4k allocation so that it can clear
> C-bit. Let's define few extra static page sized array of pvclock data.
> In the preparatory stage of CPU hotplug, use the element of this static
> array to avoid the dynamic allocation. This array will be put in
> the .data..decrypted section so that its mapped with C=0 during the boot.
>
> In non-SEV case, this static page will unused and free'd by the
> free_decrypted_mem().
>
> 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 | 22 +++++++++++++++++++---
> arch/x86/kernel/vmlinux.lds.S | 3 +++
> arch/x86/mm/init.c | 3 +++
> arch/x86/mm/mem_encrypt.c | 10 ++++++++++
> 5 files changed, 39 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> index 802b2eb..aa204af 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_hvclock __attribute__((__section__(".data..decrypted_hvclock")))
>
> #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_hvclock
>
> #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_hvclock[];
>
> #endif /* __ASSEMBLY__ */
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 376fd3a..5b88773 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -65,6 +65,13 @@ 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);
>
> +
> +/* This should cover upto 512 VCPUS (first 64 are covered by hv_clock_boot[]). */
> +#define HVC_DECRYPTED_ARRAY_SIZE \
> + ((PAGE_SIZE * 7) / sizeof(struct pvclock_vsyscall_time_info))
I think we can define the size relative to NR_CPUS rather than picking
an arbitrary number of pages, maybe with a BUILD_BUG_ON to make sure
the total size won't require a second 2mb page for __decrpyted.
#define HVC_DECRYPTED_ARRAY_SIZE \
PAGE_ALIGN((NR_CPUS - HVC_BOOT_ARRAY_SIZE) * \
sizeof(struct pvclock_vsyscall_time_info))
> +static struct pvclock_vsyscall_time_info
> + hv_clock_dec[HVC_DECRYPTED_ARRAY_SIZE] __decrypted_hvclock;
> +
> static inline struct pvclock_vcpu_time_info *this_cpu_pvti(void)
> {
> return &this_cpu_read(hv_clock_per_cpu)->pvti;
> @@ -267,10 +274,19 @@ static int kvmclock_setup_percpu(unsigned int cpu)
> return 0;
>
> /* Use the static page for the first CPUs, allocate otherwise */
> - if (cpu < HVC_BOOT_ARRAY_SIZE)
> + if (cpu < HVC_BOOT_ARRAY_SIZE) {
> p = &hv_clock_boot[cpu];
> - else
> - p = kzalloc(sizeof(*p), GFP_KERNEL);
> + } else {
> + /*
> + * When SEV is active, use the static pages from
> + * .data..decrypted_hvclock section. The pages are already
> + * mapped with C=0.
> + */
> + if (sev_active())
> + p = &hv_clock_dec[cpu - HVC_BOOT_ARRAY_SIZE];
> + else
> + p = kzalloc(sizeof(*p), GFP_KERNEL);
> + }
Personal preference, but I think an if-elif-else with a single block
comment would be easier to read.
/*
* Blah blah blah
*/
if (cpu < HVC_BOOT_ARRAY_SIZE)
p = &hv_clock_boot[cpu];
else if (sev_active())
p = &hv_clock_dec[cpu - HVC_BOOT_ARRAY_SIZE];
else
p = kzalloc(sizeof(*p), GFP_KERNEL);
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