Re: [PATCH v7 1/8] efi: Export boot-services code and data as debugfs-blobs

Hans de Goede
2019-10-09T13:18:24Z

Hi,

On 09-10-2019 15:07, Ard Biesheuvel wrote:
> On Fri, 4 Oct 2019 at 16:51, Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Sometimes it is useful to be able to dump the efi boot-services code and
>> data. This commit adds these as debugfs-blobs to /sys/kernel/debug/efi,
>> but only if efi=debug is passed on the kernel-commandline as this requires
>> not freeing those memory-regions, which costs 20+ MB of RAM.
>>
>> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>> ---
>> Changes in v5:
>> -Rename the EFI_BOOT_SERVICES flag to EFI_PRESERVE_BS_REGIONS
>>
>> Changes in v4:
>> -Add new EFI_BOOT_SERVICES flag and use it to determine if the boot-services
>> memory segments are available (and thus if it makes sense to register the
>> debugfs bits for them)
>>
>> Changes in v2:
>> -Do not call pr_err on debugfs call failures
>> ---
>> arch/x86/platform/efi/efi.c | 1 +
>> arch/x86/platform/efi/quirks.c | 4 +++
>> drivers/firmware/efi/efi.c | 53 ++++++++++++++++++++++++++++++++++
>> include/linux/efi.h | 1 +
>> 4 files changed, 59 insertions(+)
>>
>> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
>> index c202e1b07e29..847730f7e74b 100644
>> --- a/arch/x86/platform/efi/efi.c
>> +++ b/arch/x86/platform/efi/efi.c
>> @@ -232,6 +232,7 @@ int __init efi_memblock_x86_reserve_range(void)
>> efi.memmap.desc_version);
>>
>> memblock_reserve(pmap, efi.memmap.nr_map * efi.memmap.desc_size);
>> + set_bit(EFI_PRESERVE_BS_REGIONS, &efi.flags);
>
> Should we add a Kconfig symbol to opt into this behavior [set by the
> driver in question], instead of always preserving all boot services
> regions on all x86 systems?

This bit does not control anything, it merely signals that the arch early
boot EFI code keeps the boot-services code around, which is something
which the x86 code already does. Where as e.g. on arm / aarch64 this is
freed early on, this ties in with the other bits:

>
>>
>> return 0;
>> }
>> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
>> index 3b9fd679cea9..fab12ebf0ada 100644
>> --- a/arch/x86/platform/efi/quirks.c
>> +++ b/arch/x86/platform/efi/quirks.c
>> @@ -411,6 +411,10 @@ void __init efi_free_boot_services(void)
>> int num_entries = 0;
>> void *new, *new_md;
>>
>> + /* Keep all regions for /sys/kernel/debug/efi */
>> + if (efi_enabled(EFI_DBG))
>> + return;
>> +

This is the point where normally on x86 we do actually free the boot-services
code which is a lot later then on other arches. And this new code actually
does change things to keep the boot-services code *forever* but only
if EFI debugging is enabled on the kernel commandline.

This ties in with the next bit:

>> for_each_efi_memory_desc(md) {
>> unsigned long long start = md->phys_addr;
>> unsigned long long size = md->num_pages << EFI_PAGE_SHIFT;
>> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
>> index 8d3e778e988b..abba49c4c46d 100644
>> --- a/drivers/firmware/efi/efi.c
>> +++ b/drivers/firmware/efi/efi.c

<snip>

>> @@ -370,6 +420,9 @@ static int __init efisubsys_init(void)
>> goto err_remove_group;
>> }
>>
>> + if (efi_enabled(EFI_DBG) && efi_enabled(EFI_PRESERVE_BS_REGIONS))
>> + efi_debugfs_init();
>> +
>> return 0;
>>
>> err_remove_group:

Here we register the debugfs dir + files, but only when the
boot services code has been kept around, so only if the
EFI_PRESERVE_BS_REGIONS arch feature flag has been set and
EFI debugging has been requested on the kernel commandline.

IOW this patch already offers to configurability you ask for, but instead
of through a Kconfig option (which IMHO would be cumbersome) the decision
is made runtime based on the presence of efi=debug on the kernel commandline.

Regards,

Hans

>> diff --git a/include/linux/efi.h b/include/linux/efi.h
>> index bd3837022307..2a30a1bd8bdf 100644
>> --- a/include/linux/efi.h
>> +++ b/include/linux/efi.h
>> @@ -1202,6 +1202,7 @@ extern int __init efi_setup_pcdp_console(char *);
>> #define EFI_DBG 8 /* Print additional debug info at runtime */
>> #define EFI_NX_PE_DATA 9 /* Can runtime data regions be mapped non-executable? */
>> #define EFI_MEM_ATTR 10 /* Did firmware publish an EFI_MEMORY_ATTRIBUTES table? */
>> +#define EFI_PRESERVE_BS_REGIONS 11 /* Are EFI boot-services memory segments available? */
>>
>> #ifdef CONFIG_EFI
>> /*
>> --
>> 2.23.0
>>