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

[Ard Biesheuvel]

On Wed, 9 Oct 2019 at 15:59, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>
> Hi,
>
> On 09-10-2019 15:35, Ard Biesheuvel wrote:
> > On Wed, 9 Oct 2019 at 15:18, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >> On 09-10-2019 15:07, Ard Biesheuvel wrote:
> >>> On Fri, 4 Oct 2019 at 16:51, Hans de Goede <hdegoede@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> >>>> Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> >>>> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
> >>>> ---
> >>>> 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.
> >>
> >
> > I get this part. But at some point, your driver is going to expect
> > this memory to be preserved even if EFI_DBG is not set, right? My
> > question was whether we should only opt into that if such a driver is
> > enabled in the first place.
>
> Ah, I see. No even with CONFIG_EFI_EMBEDDED_FIRMWARE selected, the
> boot-services code still gets free-ed. The efi_get_embedded_fw()
> function from drivers/firmware/efi/embedded-firmware.c runs before
> efi_free_boot_services() and it memdup-s any found firmwares, so it
> does not cause the EFI boot-services code to stick around longer
> then usual.
>
> The only thing which does cause it to stick around is enabling
> EFI debugging with efi=debug, so that the various efi segments
> (not only the code-services ones) can be inspected as debugfs
> blobs.
>
> Basically this first patch of the series is independent of the
> rest. It is part of the series, because adding new
> efi_embedded_fw_desc structs to the table of firmwares to check
> for becomes a lot easier when we can easily inspect the efi
> segments and see if they contain the firmware we want.
>
>
> As for Kconfig options, the compiling of
> drivers/firmware/efi/embedded-firmware.c is controlled by
> CONFIG_EFI_EMBEDDED_FIRMWARE which is a hidden option, which
> can be selected by code which needs this. currently this is
> only selected by CONFIG_TOUCHSCREEN_DMI which is defined
> in drivers/platform/x86/Kconfig.
>

OK, thanks for clearing that up.