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    SubjectRe: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0
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    On 11/6/2017 4:01 PM, Tom Lendacky wrote:
    > On 11/6/2017 3:41 PM, H. Peter Anvin wrote:
    >> On 11/06/17 12:17, Tom Lendacky wrote:
    >>> When crosvm is used to boot a kernel as a VM, the SMP MP-table is found
    >>> at physical address 0x0. This causes mpf_base to be set to 0 and a
    >>> subsequent "if (!mpf_base)" check in default_get_smp_config() results in
    >>> the MP-table not being parsed.  Further into the boot this results in an
    >>> oops when attempting a read_apic_id().
    >>>
    >>> Add a boolean variable that is set to true when the MP-table is found.
    >>> Use this variable for testing if the MP-table was found so that even a
    >>> value of 0 for mpf_base will result in continued parsing of the MP-table.
    >>>
    >>> Reported-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
    >>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    >>
    >> Ahem... did anyone ever tell you that this is an epicly bad idea on your
    >> part?  The low megabyte of physical memory has very special meaning on
    >> x86, and deviating from the standard use of this memory is a *very*
    >> dangerous thing to do, and imposing on the kernel a "fake null pointer"
    >> requirement that exists only for the convenience of your particular
    >> brokenness is not okay.
    >>
    >>     -hpa
    >
    > That was my initial thought... what was something doing down at the start
    > of memory.  But when I looked at default_find_smp_config() it specifically
    > scans the bottom 1K for a an MP-table signature. I was hoping to get some
    > feedback as to whether this would really be an acceptable thing to do. So
    > I'm good with this patch being rejected, but the change I made in
    >
    > 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data")
    >
    > does break something that was working before.

    Btw, it was working before because instead of saving off the physical
    address as 5997efb96756 now does, it saved off a virtual address that
    pointed to physical address 0 (0xffff880000000000) and used that in the
    conditional.

    Thanks,
    Tom


    >
    > Thanks,
    > Tom
    >
    >>

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