[RFC PATCH 1/3] ARM, arm64: kvm: get rid of the bounce page
Marc Zyngier
marc.zyngier at arm.com
Thu Feb 26 08:10:50 PST 2015
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@vger.kernel.org/msg112364.html
Another question below:
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
> 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...
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