[PATCH] arm64: kernel: fix tcr_el1.t0sz restore on systems with extended idmap

Ard Biesheuvel ard.biesheuvel at linaro.org
Sat Oct 24 01:47:22 PDT 2015

Hi Lorenzo,

Thanks for the fix. I'm a bit embarrassed since this is quite
obviously broken, and I (nor anyone else, for that matter) spotted it
through testing suspend/resume on Seattle (frankly, I don't even know
if suspend/resume works currently on Seattle or if it did at the time)

On 21/10/2015, Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote:
> Commit dd006da21646 ("arm64: mm: increase VA range of identity map")
> introduced a mechanism to extend the virtual memory map range
> to support arm64 systems with system RAM located at very high offset,
> where the identity mapping used to enable/disable the MMU requires
> additional translation levels to map the physical memory at an equal
> virtual offset.
> The kernel detects at boot time the tcr_el1.t0sz value required by the
> identity mapping and sets-up the tcr_el1.t0sz register field accordingly,
> any time the identity map is required in the kernel (ie when enabling the
> MMU).
> After enabling the MMU, in the cold boot path the kernel resets the
> tcr_el1.t0sz to its default value (ie the actual configuration value for
> the system virtual address space) so that after enabling the MMU the
> memory space translated by ttbr0_el1 is restored as expected.

'After enabling the MMU' is confusing, especially since you use it
twice to describe two different points in time.

> Commit dd006da21646 ("arm64: mm: increase VA range of identity map")
> also added code to set-up the tcr_el1.t0sz value when the kernel resumes
> from low-power states with the MMU off through cpu_resume() in order to
> effectively use the identity mapping to enable the MMU but failed to add
> the code required to restore the tcr_el1.t0sz to its default value, when
> the core returns to the kernel with the MMU enabled, so that the kernel
> might end up running with tcr_el1.t0sz value set-up for the identity
> mapping which can be lower than the value required by the actual virtual
> address space, resulting in an erroneous set-up.
> This patchs adds code in the resume path that restores the tcr_el1.t0sz
> default value upon core resume, mirroring this way the cold boot path
> behaviour therefore fixing the issue.

Of course, I agree with the patch, but could you condense the commit
log, please?  I don't think we need five paragraphs to point out that
the sequence to set T0SZ to its idmap value, enable the MMU, branch
into the kernel virtual mapping and set T0SZ to its default value is
missing the final step on the resume path, and it actually took me a
while to figure out what the problem was from reading the text.

> Fixes: 95322526ef62 ("arm64: kernel: cpu_{suspend/resume} implementation")

Wrong commit id

> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>

Acked-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

> ---
>  arch/arm64/kernel/suspend.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
> index 8297d50..44ca414 100644
> --- a/arch/arm64/kernel/suspend.c
> +++ b/arch/arm64/kernel/suspend.c
> @@ -80,17 +80,21 @@ int cpu_suspend(unsigned long arg, int (*fn)(unsigned
> long))
>   if (ret == 0) {
>   /*
>   * We are resuming from reset with TTBR0_EL1 set to the
> - * idmap to enable the MMU; restore the active_mm mappings in
> - * TTBR0_EL1 unless the active_mm == &init_mm, in which case
> - * the thread entered cpu_suspend with TTBR0_EL1 set to
> - * reserved TTBR0 page tables and should be restored as such.
> + * idmap to enable the MMU; set the TTBR0 to the reserved
> + * page tables to prevent speculative TLB allocations, flush
> + * the local tlb and set the default tcr_el1.t0sz so that
> + * the TTBR0 address space set-up is properly restored.
> + * If the current active_mm != &init_mm we entered cpu_suspend
> + * with mappings in TTBR0 that must be restored, so we switch
> + * them back to complete the address space configuration
> + * restoration before returning.
>   */
> - if (mm == &init_mm)
> - cpu_set_reserved_ttbr0();
> - else
> - cpu_switch_mm(mm->pgd, mm);
> -
> + cpu_set_reserved_ttbr0();
>   flush_tlb_all();
> + cpu_set_default_tcr_t0sz();
> +
> + if (mm != &init_mm)
> + cpu_switch_mm(mm->pgd, mm);
>   /*
>   * Restore per-cpu offset before any kernel
> --
> 2.5.1

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