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    SubjectRe: [Fastboot] [PATCH] Reserving backup region for kexec based crashdumps.
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    Hi,

    > > Hi Eric,
    > >
    > > > > Hi Vivek and Eric,
    > > > >
    > > > > IMHO, why don't we swap not only the contents of the top 640K
    > > > > but also kernel working memory for kdump kernel?
    > > > >
    > > > > I guess this approach has some good points.
    > > > >
    > > > > 1.Preallocating reserved area is not mandatory at boot time.
    > > > > And the reserved area can be distributed in small pieces
    > > > > like original kexec does.
    > > > >
    > > > > 2.Special linking is not required for kdump kernel.
    > > > > Each kdump kernel can be linked in the same way,
    > > > > where the original kernel exists.
    > > > >
    > > > > Am I missing something?
    > > >
    > > > Preallocating the reserved area is largely to keep it from
    > > > being the target of DMA accesses. Since we are not able
    > > > to shutdown any of the drivers in the primary kernel running
    > > > in a normal swath of memory sounds like a good way to get
    > > > yourself stomped at the worst possible time.
    > >
    > > So what do you think my another idea?
    >
    > I have proposed it. I think ia64 already does that.
    > It has been pointed that the PowerPC kernel occasionally runs
    > with the mmu turned off. So it is not a technique the is 100%
    > portable.

    I see you have.
    And MIPS CPUs doesn't allow kernel pages to be remapped either.

    > > I think we can always make a kdump kernel mapped to the same virtual
    > > address. So we will be free from caring about the physical address
    > > where the kdump kernel is loaded.
    > >
    > > I believe the memsection functionality which LHMS project is working
    > > on would help this.
    >
    > You don't need anything fancy except to build the page tables
    > during bootup. However there are a few potential gotchas
    > with respect to using large pages, that can give 4MiB or
    > greater alignment restrictions on the kernel. Code wise
    > the gotcha is moving the kernel's .text section into what
    > is essentially the vmalloc portion of the address space.
    > For x86_64 the kernels virtual address is already decoupled from the
    > physical addresses, so it is probably easier.

    I know we can place the kernel in any address though there
    exist some exceptions.

    I know mapping kernel pages to the same virtual address only helps
    to avoid caring about physical addresses or vmalloc'ed addresses
    when linking the kernel. I think it wouldn't be bad idea in many
    architectures. I prefer it rather than linking the kernel for each
    system.

    > Most of this just results in easier management between the pieces.
    > Which is a good thing. However at the moment I don't think it
    > simplifies any of the core problems. I still need to reserve
    > a large hunk of physical address space early on before any
    > DMA transactions are setup to hold the new kernel.

    I agree that my idea is not essential at the moment.

    > So while I am happy to see patches that improve this I don't
    > actually care right now.

    ok.

    > Eric
    >

    Thanks,
    Hirokazu Takahashi.
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