Re: [RFC PATCH v2 07/26] KVM: arm64: Introduce a BSS section for use at Hyp

[Will Deacon]

On Fri, Jan 08, 2021 at 12:15:05PM +0000, Quentin Perret wrote:
> Currently, the hyp code cannot make full use of a bss, as the kernel
> section is mapped read-only.
> 
> While this mapping could simply be changed to read-write, it would
> intermingle even more the hyp and kernel state than they currently are.
> Instead, introduce a __hyp_bss section, that uses reserved pages, and
> create the appropriate RW hyp mappings during KVM init.
> 
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/sections.h |  1 +
>  arch/arm64/kernel/vmlinux.lds.S   |  7 +++++++
>  arch/arm64/kvm/arm.c              | 11 +++++++++++
>  arch/arm64/kvm/hyp/nvhe/hyp.lds.S |  1 +
>  4 files changed, 20 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h
> index 8ff579361731..f58cf493de16 100644
> --- a/arch/arm64/include/asm/sections.h
> +++ b/arch/arm64/include/asm/sections.h
> @@ -12,6 +12,7 @@ extern char __hibernate_exit_text_start[], __hibernate_exit_text_end[];
>  extern char __hyp_idmap_text_start[], __hyp_idmap_text_end[];
>  extern char __hyp_text_start[], __hyp_text_end[];
>  extern char __hyp_data_ro_after_init_start[], __hyp_data_ro_after_init_end[];
> +extern char __hyp_bss_start[], __hyp_bss_end[];
>  extern char __idmap_text_start[], __idmap_text_end[];
>  extern char __initdata_begin[], __initdata_end[];
>  extern char __inittext_begin[], __inittext_end[];
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 43af13968dfd..3eca35d5a7cf 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -8,6 +8,13 @@
>  #define RO_EXCEPTION_TABLE_ALIGN	8
>  #define RUNTIME_DISCARD_EXIT
>  
> +#define BSS_FIRST_SECTIONS				\
> +	. = ALIGN(PAGE_SIZE);				\
> +	__hyp_bss_start = .;				\
> +	*(.hyp.bss)					\

Use HYP_SECTION_NAME() here?

> +	. = ALIGN(PAGE_SIZE);				\
> +	__hyp_bss_end = .;

Should this be gated on CONFIG_KVM like the other hyp sections are? In fact,
it might be nice to define all of those together. Yeah, it means moving
things higher up in the file, but I think it will be easier to read.

>  #include <asm-generic/vmlinux.lds.h>
>  #include <asm/cache.h>
>  #include <asm/hyp_image.h>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 3ac0f3425833..51b53ca36dc5 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1770,7 +1770,18 @@ static int init_hyp_mode(void)
>  		goto out_err;
>  	}
>  
> +	/*
> +	 * .hyp.bss is placed at the beginning of the .bss section, so map that
> +	 * part RW, and the rest RO as the hyp shouldn't be touching it.
> +	 */
>  	err = create_hyp_mappings(kvm_ksym_ref(__bss_start),

I think it would be clearer to refer to __hyp_bss_start here ^^.
You could always add an ASSERT in the linker script if you want to catch
anybody adding something before the hyp bss in future.

Will