Re: [Xen-devel] Xen 4.12 panic on Thinkpad W540 with UEFI mutiboot2, efi=no-rs workarounds it

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Oct 08, 2019 at 04:19:13PM +0200, Jan Beulich wrote:
> On 08.10.2019 15:52, Marek Marczykowski-Górecki  wrote:
> > On Tue, Oct 08, 2019 at 03:08:29PM +0200, Jan Beulich wrote:
> >> On 08.10.2019 13:50, Marek Marczykowski-Górecki  wrote:
> >>> I explored it a bit more and talked with a few people doing firmware
> >>> development and few conclusions:
> >>> 1. Not calling SetVirtualAddressMap(), while technically legal, is
> >>> pretty uncommon and not recommended if you want to avoid less tested
> >>> (aka buggy) UEFI code paths.
> >>> 2. Every UEFI call before SetVirtualAddressMap() call should be done
> >>> with flat physical memory. This include SetVirtualAddressMap() call
> >>> itself. Implicitly this means such calls can legally access memory areas
> >>> not marked with EFI_MEMORY_RUNTIME.
> >>
> >> I don't think this is quite right - whether non-runtime memory may
> >> be touched depends exclusively on ExitBootServices() (not) having
> >> got called (yet).
> > 
> > That would be logical. In practice however we have evidences firmware
> > vendors have different opinion... A comment from Linux (already quoted
> > here 2 months ago):
> > 
> >     /*
> >      * The UEFI specification makes it clear that the operating system is
> >      * free to do whatever it wants with boot services code after
> >      * ExitBootServices() has been called. Ignoring this recommendation a
> >      * significant bunch of EFI implementations continue calling into boot
> >      * services code (SetVirtualAddressMap). In order to work around such
> >      * buggy implementations we reserve boot services region during EFI
> >      * init and make sure it stays executable. Then, after
> >      * SetVirtualAddressMap(), it is discarded.
> >      *
> >      * However, some boot services regions contain data that is required
> >      * by drivers, so we need to track which memory ranges can never be
> >      * freed. This is done by tagging those regions with the
> >      * EFI_MEMORY_RUNTIME attribute.
> >      *
> >      * Any driver that wants to mark a region as reserved must use
> >      * efi_mem_reserve() which will insert a new EFI memory descriptor
> >      * into efi.memmap (splitting existing regions if necessary) and tag
> >      * it with EFI_MEMORY_RUNTIME.
> >      */
> 
> But you realize that the comment specifically talks about
> the call tree underneath SetVirtualAddressMap() being the violator.
> As long as we don't call this function, we're unaffected as far as
> this comment goes.

Well, this very thread proves it isn't only about
SetVirtualAddressMap(). I _guess_ it's about calls before
SetVirtualAddressMap() returns (which supposedly do some cleanups). In
Linux case, it is only that one call.

> > Regardless of SetVirtualAddressMap() discussion, I propose to
> > automatically map boot services code/data, to make Xen work on more
> > machines (even if _we_ consider those buggy). 
> 
> I remain on my prior position: Adding command line triggerable
> workarounds for such cases is fine. Defaulting to assume buggy
> firmware is acceptable only if this means no extra penalty to
> systems with conforming firmware. Hence, for the case at hand,
> I object to doing this automatically; we already have the
> /mapbs workaround in place to deal with the case when xen.efi
> is used. Judging from the title here there may need to be an
> addition to also allow triggering this from the MB2 boot path.

What about mirroring Linux behavior? I.e. mapping those regions for the
SetVirtualAddressMap() time (when enabled) and unmapping after - unless
tagged with EFI_MEMORY_RUNTIME? 
Similarly to Andrew, I'd really prefer for Xen to work out of the box,
with as little as possible manual tweaks needed.

> >>> Then I've tried a different approach: call SetVirtualAddressMap(), but
> >>> with an address map that tries to pretend physical addressing (the code
> >>> under #ifndef USE_SET_VIRTUAL_ADDRESS_MAP). This mostly worked, I needed
> >>> only few changes:
> >>>  - set VirtualStart back to PhysicalStart in that memory map (it was set
> >>>    to directmap)
> >>>  - map boot services (at least for the SetVirtualAddressMap() call time,
> >>>    but haven't tried unmapping it later)
> >>>  - call SetVirtualAddressMap() with that "1:1" map in place, using
> >>>    efi_rs_enter/efi_rs_leave.
> >>>
> >>> This fixed the issue for me, now runtime services do work even without
> >>> disabling ExitBootServices() call. And without any extra
> >>> platform-specific command line arguments. And I think it also shouldn't break
> >>> kexec, since it uses 1:1-like map, but I haven't tried. One should
> >>> simply ignore EFI_UNSUPPORTED return code (I don't know how to avoid the
> >>> call at all after kexec).
> >>
> >> That's the point - it can't be avoided, and hence it failing is not
> >> an option. Or else there needs to be a protocol telling kexec what
> >> it is to expect, and that it in particular can't change the virtual
> >> address map to its liking. Back at the time when I put together the
> >> EFI booting code, no such protocol existed, and hence calling
> >> SetVirtualAddressMap() was not an option. I have no idea whether
> >> things have changed in the meantime.
> > 
> > Hmm, how is it different from the current situation? Not calling
> > SetVirtualAddressMap() means UEFI will not be notified about new address
> > map. It does _not_ mean it will use 1:1 map, it will use what was
> > previously set.
> 
> What do you mean by "previously set"? SetVirtualAddressMap() can be
> invoked only once during a given session (i.e. without intervening
> boot). How would other than a 1:1 map have got set?

Like, in the very next sentence of my answer:

> > What if Xen was kexec'ed from Linux?

> > In Linux case, it looks like it passes around the EFI memory map using
> > some Linux-specific mechanism, but I don't find it particularly
> > appealing option.
> 
> Indeed.
> 
> > What about something in between: make this SetVirtualAddressMap() call
> > compile-time option (kconfig), depending on !CONFIG_KEXEC ? And when
> > enabled, properly handle SetVirtualAddressMap() failure.
> 
> What is "proper handling" here?

Logging the error and either panic or disabling runtime services (I tend
to the latter).

> > I my case,
> > where I do care about supporting various UEFI implementations, I don't
> > need kexec support. And apparently people carrying about kexec don't
> > have problems with lack of SetVirtualAddressMap(), so that would be
> > win-win, no?
> 
> Allowing SetVirtualAddressMap() when !KEXEC would be fine with me.
> The fly in the ointment here is that we'd prefer not to have such
> Kconfig options (at least not without EXPERT qualifier), as
> (security) supporting all the possible combinations would be a
> nightmare. If an EXPERT dependency is okay with you, then I'll be
> looking forward to your patch.

EXPERT is fine with me.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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