Re: [PATCH v2 1/2] efi: esrt: use memremap not ioremap to access ESRT table in memory

[Dave Young <dyoung-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 02/18/16 at 03:21pm, Ard Biesheuvel wrote:
> On 18 February 2016 at 15:15, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> > On Thu, 18 Feb, at 02:44:02PM, Ard Biesheuvel wrote:
> >> On 18 February 2016 at 14:43, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> >> > On Thu, 18 Feb, at 02:29:32PM, Ard Biesheuvel wrote:
> >> >> On 18 February 2016 at 14:28, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> >> >> > On Thu, 18 Feb, at 01:16:05PM, Ard Biesheuvel wrote:
> >> >> >> On 18 February 2016 at 11:44, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> >> >> >> > On Mon, 15 Feb, at 12:32:32PM, Ard Biesheuvel wrote:
> >> >> >> >> On ARM and arm64, ioremap() and memremap() are not interchangeable like
> >> >> >> >> on x86, and the use of ioremap() on ordinary RAM is typically flagged
> >> >> >> >> as an error if the memory region being mapped is also covered by the
> >> >> >> >> linear mapping, since that would lead to aliases with conflicting
> >> >> >> >> cacheability attributes.
> >> >> >> >>
> >> >> >> >> Since what we are dealing with is not an I/O region with side effects,
> >> >> >> >> using ioremap() here is arguably incorrect anyway, so let's replace
> >> >> >> >> it with memremap instead. Also add a missing unmap on the success path,
> >> >> >> >> and drop a memblock_remove() call which does not belong here, this far
> >> >> >> >> into the boot sequence.
> >> >> >> >>
> >> >> >> >> Cc: Peter Jones <pjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> >> >> >> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >> >> >> >> ---
> >> >> >> >>  drivers/firmware/efi/esrt.c | 16 ++++++++--------
> >> >> >> >>  1 file changed, 8 insertions(+), 8 deletions(-)
> >> >> >> >>
> >> >> >> >
> >> >> >> > [...]
> >> >> >> >
> >> >> >> >> @@ -432,8 +434,6 @@ static int __init esrt_sysfs_init(void)
> >> >> >> >>       if (error)
> >> >> >> >>               goto err_cleanup_list;
> >> >> >> >>
> >> >> >> >> -     memblock_remove(esrt_data, esrt_data_size);
> >> >> >> >> -
> >> >> >> >>       pr_debug("esrt-sysfs: loaded.\n");
> >> >> >> >>
> >> >> >> >>       return 0;
> >> >> >> >
> >> >> >> > Shouldn't we be replacing memblock_remove() with free_bootmem_late()?
> >> >> >> > The original ESRT region is still reserved at this point, so we should
> >> >> >> > do our best to release it to the page allocator.
> >> >> >>
> >> >> >> I'd rather we keep it reserved. That way, the config table entry still
> >> >> >> points to something valid, which could be useful for kexec(), I think?
> >> >> >> At least, that is how I intended to handle config tables on ARM ...
> >> >> >
> >> >> > If we're going to reserve it why do we need to copy the data out at
> >> >> > all in esrt_sysfs_init()?
> >> >>
> >> >> Excellent question. I don't think there is any point to doing that.
> >> >
> >> > ... Unless the data is contained in an EFI Boot Services region ;-)
> >> >
> >> > Peter?
> >>
> >> Yes, it usually is. Is that a problem?
> >
> > Yes, we free the Boot Services regions before hitting userspace on
> > x86, see efi_free_boot_services(). We do this map/copy/unmap trick in
> > the ACPI BGRT driver for that reason.
> >
> > The Boot Services regions can be many gigabytes in size, which makes
> > leaving them alone impractical.
> >
> > For kexec on x86 we simply discard the BGRT table, which isn't the end
> > of the world because who really needs access to the BGRT image on
> > kexec reboot? However, I can see the value of preserving the ESRT.
> >
> > I guess we've got two options, 1) copy out the chunks of Boot Services
> > regions we're interested in and rewrite the EFI tables to point at
> > these new allocations and free/discard all of the original Boot
> > Services regions or 2) only selectively free the Boot Services
> > regions.
> >
> > I've always stayed clear of 2) in case there exists cross-region
> > references in the data that isn't obvious. I'd like to think that
> > would never happen, but, you know, dragons lurk here, etc.
> >
> > Though actually, now I think about it, cross-region references can't
> > possibly exist because they'd cause issues with the current code.
> >
> > So maybe the best solution is actually 2), where we preserve the Boot
> > Services regions if any of the drivers (ESRT, BGRT) request them but
> > free all the others?

It is a good idea so that drivers can add their useful sections to
a list or an array, they can avoid another copy of the memory also.

> >
> > What are the lifetime rules for Boot Services regions on arm*?
> 
> We treat all Boot Services regions like Loader Code/Data or free
> regions: it is all recorded in memblock as usable memory, and only the
> regions that are explicitly reserved are protected from further
> general use.
> 
> I am currently looking into the memory attribute table, and the use
> case is very similar. It would be very useful from our pov to simply
> memblock_reserve() the region right after having called
> efi_config_parse_tables(), and actually consume its data when we get
> around to it later. The ESRT handling is already split down the middle
> in the same way.

A question is how can make a general way for both x86 and arm*.

Maybe change the efi_free_boot_mem to something like efi_clean_boot_mem?
It can first call efi_free_boot_mem, then reserve the ranges to be reserved
again, but it sounds odd though.

Thanks
Dave