[PATCH] arm64: suspend: avoid potential TLB conflict

[James Morse]

Hi Mark,

~s/suspend/hibernate/ in the subject?

On 08/08/16 11:10, Mark Rutland wrote:
> In create_safe_exec_page we install a set of global mappings in TTBR0,
> then subsequently invalidate TLBs. While TTBR0 points at the zero page,
> and the TLBs should be free of stale global entries, we may have stale
> ASID-tagged entries (e.g. from the EFI runtime services mappings) for
> the same VAs. Per the ARM ARM these ASID-tagged entries may conflict
> with newly-allocated global entries, and we must follow a
> Break-Before-Make approach to avoid issues resulting from this.
> 
> This patch reworks create_safe_exec_page to invalidate TLBs while the
> zero page is still in place, ensuring that there are no potential
> conflicts when the new TTBR0 value is installed. As a single CPU is
> online while this code executes, we do not need to perform broadcast TLB
> maintenance, and can call local_flush_tlb_all(), which also subsumes
> some barriers. The remaining assembly is converted to use write_sysreg()
> and isb().
> 
> Other than this, we safely manipulate TTBRs in the hibernate dance. The
> code we install as part of the new TTBR0 mapping (the hibernated
> kernel's swsusp_arch_suspend_exit) installs a zero page into TTBR1,
> invalidates TLBs, then installs its preferred value. Upon being restored
> to the middle of swsusp_arch_suspend, the new image will call
> __cpu_suspend_exit, which will call cpu_uninstall_idmap, installing the
> zero page in TTBR0 and invalidating all TLB entries.

> diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
> index 21ab5df..c59d3e4 100644
> --- a/arch/arm64/kernel/hibernate.c
> +++ b/arch/arm64/kernel/hibernate.c
> @@ -35,6 +35,7 @@
>  #include <asm/sections.h>
>  #include <asm/smp.h>
>  #include <asm/suspend.h>
> +#include <asm/sysreg.h>
>  #include <asm/virt.h>
>  
>  /*
> @@ -217,12 +218,16 @@ static int create_safe_exec_page(void *src_start, size_t length,
>  	set_pte(pte, __pte(virt_to_phys((void *)dst) |
>  			 pgprot_val(PAGE_KERNEL_EXEC)));
>  
> -	/* Load our new page tables */
> -	asm volatile("msr	ttbr0_el1, %0;"
> -		     "isb;"
> -		     "tlbi	vmalle1is;"
> -		     "dsb	ish;"
> -		     "isb" : : "r"(virt_to_phys(pgd)));
> +	/*
> +	 * Load our new page tables. TTBR0 currently points to the zero page,

fe12c00d21bb ("PM / hibernate: Introduce test_resume mode for hibernation") came
in with the merge window, this does a suspend followed by a resume with the user
page tables still loaded in ttbr0_el1.

So now we need to call cpu_set_reserved_ttbr0() in here to make this true/safe.


> +	 * and the TLBs should be free of global entries, but may contain stale
> +	 * ASID-tagged entries (e.g. from the EFI runtime services). A strict
> +	 * BBM approach requires that we destroy these before installing
> +	 * overlapping global mappings.
> +	 */
> +	local_flush_tlb_all();
> +	write_sysreg(virt_to_phys(pgd), ttbr0_el1);
> +	isb();
>  
>  	*phys_dst_addr = virt_to_phys((void *)dst);

and it even looks better!

If you think they're useful:
Tested-by: James Morse <james.morse at arm.com>
Acked-by: James Morse <james.morse at arm.com>


Thanks!

James