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    SubjectRe: [PATCH 0/2] x86/microcode: support for microcode update in Xen dom0
    On Fri, Jan 28, 2011 at 04:26:52PM -0800, Jeremy Fitzhardinge wrote:
    > From: Jeremy Fitzhardinge <>
    > Hi all,
    > I'm proposing this for 2.6.39.
    > This series adds a new "Xen" microcode update type, in addition to
    > Intel and AMD.
    > The Xen hypervisor is responsible for performing the actual microcode
    > update (since only it knows what physical CPUs are in the system and
    > has sufficient privilege to access them), but it requires the dom0
    > kernel to provide the actual microcode update data.
    > Xen update mechanism is uniform independent of the CPU type, but the
    > driver must know where to find the data file, which depends on the CPU
    > type. And since the update hypercall updates all CPUs, we only need
    > to execute it once on any CPU - but for simplicity it just runs it only
    > on (V)CPU 0.

    I don't like this for several reasons:

    - ucode should be loaded as early as possible and the perfect place
    for that should be the hypervisor and not the dom0 kernel. But I'm not
    that familiar with xen design and I guess it would be pretty hard to
    do. (Btw, x86 bare metal could also try to improve the situation there
    but that's also hard due to the design of the firmware loader (needs

    - you're adding a microcode_xen.c as if this is another hw vendor and
    you're figuring out which is the proper firmware image based on the
    vendor, then you load it and then do the hypercall. This is duplicating
    code and inviting future bugs. Everytime the hw vendors decide to change
    something to their fw loading mechanism, xen needs to be updated. Also,
    you don't do any sanity checks to the ucode image as the vendor code
    does which is inacceptable.

    What it should do instead, is call into the hw vendor-specific ucode
    loading mechanism and do all the image loading/verification there. The
    only thing you'd need to supply is a xen-specific ->apply_microcode()
    doing the hypercall _after_ the ucode image has been verified and is
    good to go. I'm guessing the XENPF_microcode_update does call into the
    hypervisor and calls a xen-specific ucode update mechanism based on the
    vendor instead of using the vendor-supplied code...



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