Subject: [RFC PATCH 1/3] ARM, arm64: kvm: get rid of the bounce page

From: [email-masked] (Marc Zyngier)
Date: Thu, 26 Feb 2015 16:10:50 +0000

On 26/02/15 15:29, Ard Biesheuvel wrote:
> The HYP init bounce page is a runtime construct that ensures that the
> HYP init code does not cross a page boundary. However, this is something
> we can do perfectly well at build time, by aligning the code appropriately.
> 
> For arm64, we just align to 4 KB, and enforce that the code size is less
> than 4 KB, regardless of the chosen page size.
> 
> For ARM, the whole code is less than 256 bytes, so we tweak the linker
> script to align at a power of 2 upper bound of the code size
> 
> Note that this also fixes a benign off-by-one error in the original bounce
> page code, where a bounce page would be allocated unnecessarily if the code
> was exactly 1 page in size.

I really like this simplification. Can you please check that it still
work on 32bit with this patch from Arnd?

https://www.mail-archive.com/kvm at vger.kernel.org/msg112364.html

Another question below:

> Signed-off-by: Ard Biesheuvel <[email-masked]>
> ---
>  arch/arm/kernel/vmlinux.lds.S   | 12 +++++++++---
>  arch/arm/kvm/init.S             | 11 +++++++++++
>  arch/arm/kvm/mmu.c              | 42 +++++------------------------------------
>  arch/arm64/kernel/vmlinux.lds.S | 18 ++++++++++++------
>  4 files changed, 37 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index b31aa73e8076..8179d3903dee 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -23,7 +23,7 @@
>  	VMLINUX_SYMBOL(__idmap_text_start) = .;				\
>  	*(.idmap.text)							\
>  	VMLINUX_SYMBOL(__idmap_text_end) = .;				\
> -	. = ALIGN(32);							\
> +	. = ALIGN(1 << __hyp_idmap_align_order);			\
>  	VMLINUX_SYMBOL(__hyp_idmap_text_start) = .;			\
>  	*(.hyp.idmap.text)						\
>  	VMLINUX_SYMBOL(__hyp_idmap_text_end) = .;
> @@ -346,8 +346,14 @@ SECTIONS
>   */
>  ASSERT((__proc_info_end - __proc_info_begin), "missing CPU support")
>  ASSERT((__arch_info_end - __arch_info_begin), "no machine record defined")
> +
>  /*
> - * The HYP init code can't be more than a page long.
> + * The HYP init code can't be more than a page long,
> + * and should not cross a page boundary.
>   * The above comment applies as well.
>   */
> -ASSERT(((__hyp_idmap_text_end - __hyp_idmap_text_start) <= PAGE_SIZE), "HYP init code too big")
> +ASSERT(((__hyp_idmap_text_end - 1) & PAGE_MASK) -
> +	(__hyp_idmap_text_start & PAGE_MASK) == 0,
> +	"HYP init code too big or unaligned")
> +ASSERT(__hyp_idmap_size <= (1 << __hyp_idmap_align_order),
> +	"__hyp_idmap_size should be <= (1 << __hyp_idmap_align_order)")
> diff --git a/arch/arm/kvm/init.S b/arch/arm/kvm/init.S
> index 3988e72d16ff..7a279bc8e0e1 100644
> --- a/arch/arm/kvm/init.S
> +++ b/arch/arm/kvm/init.S
> @@ -157,3 +157,14 @@ target:	@ We're now in the trampoline code, switch page tables
>  __kvm_hyp_init_end:
>  
>  	.popsection
> +
> +	/*
> +	 * When making changes to this file, make sure that the value of
> +	 * __hyp_idmap_align_order is updated if the size of the code ends up
> +	 * exceeding (1 << __hyp_idmap_align_order). This helps ensure that the
> +	 * code never crosses a page boundary, without wasting too much memory
> +	 * on aligning to PAGE_SIZE.
> +	 */
> +	.global	__hyp_idmap_size, __hyp_idmap_align_order
> +	.set	__hyp_idmap_size, __kvm_hyp_init_end - __kvm_hyp_init
> +	.set	__hyp_idmap_align_order, 8

Is there a way to generate this __hyp_idmap_align_order automatically?
We're already pretty close to this 8 bit limit...

> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 3e6859bc3e11..42a24d6b003b 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -37,7 +37,6 @@ static pgd_t *boot_hyp_pgd;
>  static pgd_t *hyp_pgd;
>  static DEFINE_MUTEX(kvm_hyp_pgd_mutex);
>  
> -static void *init_bounce_page;
>  static unsigned long hyp_idmap_start;
>  static unsigned long hyp_idmap_end;
>  static phys_addr_t hyp_idmap_vector;
> @@ -405,9 +404,6 @@ void free_boot_hyp_pgd(void)
>  	if (hyp_pgd)
>  		unmap_range(NULL, hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
>  
> -	free_page((unsigned long)init_bounce_page);
> -	init_bounce_page = NULL;
> -
>  	mutex_unlock(&kvm_hyp_pgd_mutex);
>  }
>  
> @@ -1498,39 +1494,11 @@ int kvm_mmu_init(void)
>  	hyp_idmap_end = kvm_virt_to_phys(__hyp_idmap_text_end);
>  	hyp_idmap_vector = kvm_virt_to_phys(__kvm_hyp_init);
>  
> -	if ((hyp_idmap_start ^ hyp_idmap_end) & PAGE_MASK) {
> -		/*
> -		 * Our init code is crossing a page boundary. Allocate
> -		 * a bounce page, copy the code over and use that.
> -		 */
> -		size_t len = __hyp_idmap_text_end - __hyp_idmap_text_start;
> -		phys_addr_t phys_base;
> -
> -		init_bounce_page = (void *)__get_free_page(GFP_KERNEL);
> -		if (!init_bounce_page) {
> -			kvm_err("Couldn't allocate HYP init bounce page\n");
> -			err = -ENOMEM;
> -			goto out;
> -		}
> -
> -		memcpy(init_bounce_page, __hyp_idmap_text_start, len);
> -		/*
> -		 * Warning: the code we just copied to the bounce page
> -		 * must be flushed to the point of coherency.
> -		 * Otherwise, the data may be sitting in L2, and HYP
> -		 * mode won't be able to observe it as it runs with
> -		 * caches off at that point.
> -		 */
> -		kvm_flush_dcache_to_poc(init_bounce_page, len);
> -
> -		phys_base = kvm_virt_to_phys(init_bounce_page);
> -		hyp_idmap_vector += phys_base - hyp_idmap_start;
> -		hyp_idmap_start = phys_base;
> -		hyp_idmap_end = phys_base + len;
> -
> -		kvm_info("Using HYP init bounce page @%lx\n",
> -			 (unsigned long)phys_base);
> -	}
> +	/*
> +	 * We rely on the linker script to ensure at build time that the HYP
> +	 * init code does not cross a page boundary.
> +	 */
> +	BUG_ON((hyp_idmap_start ^ (hyp_idmap_end - 1)) & PAGE_MASK);
>  
>  	hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, hyp_pgd_order);
>  	boot_hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, hyp_pgd_order);
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 5d9d2dca530d..17383c257a7d 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -23,10 +23,14 @@ jiffies = jiffies_64;
>  
>  #define HYPERVISOR_TEXT					\
>  	/*						\
> -	 * Force the alignment to be compatible with	\
> -	 * the vectors requirements			\
> +	 * Align to 4K so that				\
> +	 * a) the HYP vector table is at its minimum	\
> +	 *    alignment of 2048 bytes			\
> +	 * b) the HYP init code will not cross a page	\
> +	 *    boundary if its size does not exceed	\
> +	 *    4K (see related ASSERT() below)		\
>  	 */						\
> -	. = ALIGN(2048);				\
> +	. = ALIGN(SZ_4K);				\
>  	VMLINUX_SYMBOL(__hyp_idmap_text_start) = .;	\
>  	*(.hyp.idmap.text)				\
>  	VMLINUX_SYMBOL(__hyp_idmap_text_end) = .;	\
> @@ -163,10 +167,12 @@ SECTIONS
>  }
>  
>  /*
> - * The HYP init code can't be more than a page long.
> + * The HYP init code can't be more than a page long,
> + * and should not cross a page boundary.
>   */
> -ASSERT(((__hyp_idmap_text_start + PAGE_SIZE) > __hyp_idmap_text_end),
> -       "HYP init code too big")
> +ASSERT(((__hyp_idmap_text_end - 1) & ~(SZ_4K - 1)) -
> +	(__hyp_idmap_text_start & ~(SZ_4K - 1)) == 0,
> +	"HYP init code too big or unaligned")
>  
>  /*
>   * If padding is applied before .head.text, virt<->phys conversions will fail.
> 

Otherwise looks pretty good.

	M.
-- 
Jazz is not dead. It just smells funny...