Re: [PATCH v4 2/2] Support ESRT in Xen dom0

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Sep 30, 2022 at 06:36:11PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 01:02, Demi Marie Obenour
> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > fwupd requires access to the EFI System Resource Table (ESRT) to
> > discover which firmware can be updated by the OS.  Currently, Linux does
> > not expose the ESRT when running as a Xen dom0.  Therefore, it is not
> > possible to use fwupd in a Xen dom0, which is a serious problem for e.g.
> > Qubes OS.
> >
> > Before Xen 4.17, this was not fixable due to hypervisor limitations.
> > The UEFI specification requires the ESRT to be in EfiBootServicesData
> > memory, which Xen will use for whatever purposes it likes.  Therefore,
> > Linux cannot safely access the ESRT, as Xen may have overwritten it.
> >
> > Starting with Xen 4.17, Xen checks if the ESRT is in EfiBootServicesData
> > or EfiRuntimeServicesData memory.  If the ESRT is in EfiBootServicesData
> > memory, Xen replaces the ESRT with a copy in memory that it has
> > reserved.  Such memory is currently of type EFI_RUNTIME_SERVICES_DATA,
> > but in the future it will be of type EFI_ACPI_RECLAIM_MEMORY.  This
> > ensures that the ESRT can safely be accessed by the OS.
> >
> > When running as a Xen dom0, use the new
> > xen_config_table_memory_region_max() function to determine if Xen has
> > reserved the ESRT and, if so, find the end of the memory region
> > containing it.  This allows programs such as fwupd which require the
> > ESRT to run under Xen, and so makes fwupd support in Qubes OS possible.
> >
> > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> 
> Why do we need this patch? I'd expect esrt_table_exists() to return
> false when patch 1/2 is applied.

efi_enabled(EFI_MEMMAP) is false under Xen, so there needs to be an
alternative way to get the end of the memory region containing the ESRT.
That is what this patch provides.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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