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

Ard Biesheuvel ard.biesheuvel at linaro.org
Mon Oct 26 06:45:11 PDT 2015

On 26 October 2015 at 19:55, Lorenzo Pieralisi
<lorenzo.pieralisi at arm.com> wrote:
> Hi Ard,
> On Sat, Oct 24, 2015 at 10:47:22AM +0200, Ard Biesheuvel wrote:
>> 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)
> You could not spot it by testing on Seattle, it does not implement PM yet,
> I spotted it by code inspection while reviewing James' suspend-to-disk
> patches, there is nothing to be embarassed about, you paved the way
> to fix it.

Ah ok, that is good to hear.

>> 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.
> I will sum up this commit log, it is ways too long and not clear
> at times.


>> > 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.
> Agreed.
>> > Fixes: 95322526ef62 ("arm64: kernel: cpu_{suspend/resume} implementation")
>> Wrong commit id
> Technically speaking I am not fixing
> Commit dd006da21646 ("arm64: mm: increase VA range of identity map")
> I am fixing the commit I reported, because it never worked on platforms
> requiring extended idmap, if anything, your commit dd006da21646 gave
> CPUidle states and suspend2RAM a chance to work on those platforms,
> and this patch completes the implementation.
> To make things simpler I can report:
> Fixes: dd006da21646 ("arm64: mm: increase VA range of identity map")
> as long as we all agree on the above.

This patch cannot be applied to any stable tree that does not have
dd006da21646, since that commit introduces cpu_set_default_tcr_t0sz(),
and your patch introduces a call to that function. However, the
context does not reflect that, and the patch may apply cleanly to
older kernels only to break at build time. So to make the lives of
stable kernel maintainers easier, it is better to use the latter IMO.

>> > 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>
> Thanks, I will prepare a v2 and ask Catalin to merge it at -rc1, it
> conflicts with patches currently on arm64 for-next/core we can wait till
> -rc1.



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