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Date:   Tue, 27 Jun 2023 09:37:46 +0100
From:   Matthew Garrett <mjg59@srcf.ucam.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi@vger.kernel.org, Daniel Kiper <dkiper@net-space.pl>,
        Glenn Washburn <development@efficientek.com>
Subject: Re: [PATCH] efi/libstub: Disable PCI DMA before grabbing the EFI
 memory map
Message-ID: <20230627083746.GA21374@srcf.ucam.org>
References: <20230627074132.1016795-1-ardb@kernel.org>
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On Tue, Jun 27, 2023 at 10:32:36AM +0200, Ard Biesheuvel wrote:
> On Tue, 27 Jun 2023 at 10:17, Matthew Garrett <mjg59@srcf.ucam.org> wrote:
> >
> > On Tue, Jun 27, 2023 at 10:14:16AM +0200, Ard Biesheuvel wrote:
> >
> > > Not quite. It should only fail the first time if the memory map
> > > changed since the last call to GetMemoryMap(), and normally, this will
> > > only happen if some kind of asynchronous event was triggered after
> > > GetMemoryMap() but before ExitBootServices(). (This is why calling
> > > ExitBootServices() at most twice should always suffice: the first call
> > > disables the timer interrupt, so the second time around, no events
> > > will fire in the mean time)
> >
> > Can't driver shutdown code also end up altering it?
> 
> Yes, but doing so violates the UEFI spec:
> EVT_SIGNAL_EXIT_BOOT_SERVICES is documented as not permitting the use
> of memory allocation services, either directly or indirectly (via the
> use of other external code that might use them)

Maybe people have become better at observing that restriction! Anyway, 
feel free to ignore my nit in that case.