On Wed, Oct 01, 2014 at 01:06:39PM +0100, Matt Fleming wrote:
> From: Matt Fleming <matt.fleming@xxxxxxxxx>
>
> There are some circumstances that call for trying to write an EFI
> variable in a non-blocking way. One such scenario is when writing pstore
> data in efi_pstore_write() via the pstore_dump() kdump callback.
>
> Now that we have an EFI runtime spinlock we need a way of aborting if
> there is contention instead of spinning, since when writing pstore data
> from the kdump callback, the runtime lock may already be held by the CPU
> that's running the callback if we crashed in the middle of an EFI
> variable operation.
>
> The situation is sufficiently special that a new EFI variable operation
> is warranted.
>
> Introduce ->set_variable_nonblocking() for this use case. It is an
> optional EFI backend operation, and need only be implemented by those
> backends that usually acquire locks to serialize access to EFI
> variables, as is the case for virt_efi_set_variable() where we now grab
> the EFI runtime spinlock.
>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
> Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxx>
> ---
> drivers/firmware/efi/runtime-wrappers.c | 19 +++++++++++++
> drivers/firmware/efi/vars.c | 47 +++++++++++++++++++++++++++++++++
> include/linux/efi.h | 6 +++++
> 3 files changed, 72 insertions(+)
>
> diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c
> index 9694cba665c4..4349206198b2 100644
> --- a/drivers/firmware/efi/runtime-wrappers.c
> +++ b/drivers/firmware/efi/runtime-wrappers.c
> @@ -200,6 +200,24 @@ static efi_status_t virt_efi_set_variable(efi_char16_t *name,
> return status;
> }
>
> +static efi_status_t
> +virt_efi_set_variable_nonblocking(efi_char16_t *name, efi_guid_t *vendor,
> + u32 attr, unsigned long data_size,
> + void *data)
> +{
> + unsigned long flags;
> + efi_status_t status;
> +
> + if (!spin_trylock_irqsave(&efi_runtime_lock, flags))
> + return EFI_NOT_READY;
> +
> + status = efi_call_virt(set_variable, name, vendor, attr, data_size,
> + data);
> + spin_unlock_irqrestore(&efi_runtime_lock, flags);
> + return status;
> +}
If you want to have this usable from NMI context, you need to convert
efi_runtime_lock to a raw_spinlock_t.
Also, it would probably be a good idea to have some selftest like thing
that actually calls this from NMI context, right?
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