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    SubjectRe: [PATCH v3 13/21] x86/virt/tdx: Allocate and set up PAMTs for TDMRs
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    On 5/1/22 22:59, Kai Huang wrote:
    > On Fri, 2022-04-29 at 07:20 -0700, Dave Hansen wrote:
    > How about adding below in the changelog:
    >
    > "
    > However using alloc_contig_pages() to allocate large physically contiguous
    > memory at runtime may fail. The larger the allocation, the more likely it is to
    > fail. Due to the fragmentation, the kernel may need to move pages out of the
    > to-be-allocated contiguous memory range but it may fail to move even the last
    > stubborn page. A good way (although not foolproof) is to launch a TD VM early
    > in boot to get PAMTs allocated before memory gets fragmented or consumed.
    > "

    Better, although it's getting a bit off topic for this changelog.

    Just be short and sweet:

    1. the allocation can fail
    2. Launch a VM early to (badly) mitigate this
    3. the only way to fix it is to add a boot option


    >>>>> + /*
    >>>>> + * One TDMR must cover at least one (or partial) RAM entry,
    >>>>> + * otherwise it is kernel bug. WARN_ON() in this case.
    >>>>> + */
    >>>>> + if (WARN_ON_ONCE((start >= end) || start >= TDMR_END(tdmr)))
    >>>>> + return 0;
    >>
    >> This really means "no RAM found for this TDMR", right? Can we say that,
    >> please.
    >
    > OK will add it. How about:
    >
    > /*
    > * No RAM found for this TDMR. WARN() in this case, as it
    > * cannot happen otherwise it is a kernel bug.
    > */

    The only useful information in that comment is the first sentence. The
    jibberish about WARN() is patently obvious from the next two lines of code.

    *WHY* can't this happen? How might it have actually happened?

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