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    SubjectRe: Fw: Re: [PATCH 1/1] EFI: Fix gdt load

    I fight against a new EFI bug. With 512MB ram the iMac boots fine, but when i add
    extra 512MB to have 1GB ram the kernel stops booting. Still didn't found the problem.
    I think it have something to do with the efi_memmap_walk funtction. The major problem
    in debuging the problem is that i have no video out at this boot stage. I only can
    test it with a machine reboot code.


    Edgar (gimli) Hucek

    Zachary Amsden wrote:
    > Andrew Morton wrote:
    >> Doh. Too many Zachs.
    >> Begin forwarded message:
    >> Date: Sat, 4 Mar 2006 18:43:19 -0800
    >> From: Andrew Morton <>
    >> To: Edgar Hucek <>
    >> Cc:, "Zach, Yoav" <>,
    >> Matt Domsch <>
    >> Subject: Re: [PATCH 1/1] EFI: Fix gdt load
    >> Edgar Hucek <> wrote:
    >>> This patch makes the kernel bootable again on ia32 EFI systems.
    >> Argh, thanks. I'll move the per_cpu() call inside the lock, just in case
    >> we happen to be running preemptibly there.
    >> Zach, Matt: please review, test and ack asap?
    > Ok, that was subtle. It took me 10 minutes staring at this code to
    > notice the extra __pa and __va in the load_gdt call. Actually, by sheer
    > coincidence, the first one was actually still correct. Normally, this
    > code would just totally blow up, but you've just identity mapped virtual
    > and physical addresses. The second one will blow up after the EFI call
    > without the fix.
    > Unfortunately, I can't test EFI; I have no machines here that are EFI
    > capable.
    > This code has always confused me, though. Why do we do this crazy hack
    > to begin with? The crazy hack is not remapping the GDT in physical
    > space, or simulating non-paging memory with paging enabled - that is
    > completely normal. But why do we muck with the GDT for CPU zero instead
    > of the current CPU? If the EFI code decides to reload FS or GS, we have
    > now leaked the user FS or GS from CPU zero onto the current CPU, and I
    > see no code here which restricts EFI to run on the BSP. This will break
    > userspace TLS programs. Of course, I have no evidence that EFI will
    > reload FS or GS, but it must be doing something with segmentation, or
    > you would not have needed to reload the GDT.
    > Second, there is another bug in this code as well. Why do we care if
    > PSE is enabled when identity mapping virtual to physical space? PSE has
    > _nothing_ to do with this. You are copying top level page ranges, which
    > are the same size, with or without PSE. We should be checking if PAE is
    > enabled, and we shouldn't even need to check, since it will either be
    > compiled in or not. This code is scarily just quite lucky that the
    > kernel is small enough to fit.
    > For PAE mode, PSE is always going to be enabled (I believe), so you end
    > up remapping 1GB of virtual space into physical space. For non-PAE, PSE
    > may or may not be enabled, in which case, you end up remapping either
    > 4MB or 8MB of the kernel virtual address space back at zero.
    > I don't believe 4MB is enough to make sure all of the per-cpu variables
    > can be safely referenced, although I could be wrong. So if there are
    > EFI machines out there with processors installed that have no PSE
    > support, and the kernel gets large enough, this code blows up again. I
    > actually think that is quite likely as EFI becomes more prevalent and
    > older core processors continue to be made for the embedded market.
    > Zach

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