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    SubjectRe: [PATCH v5 12/22] x86/virt/tdx: Convert all memory regions in memblock to TDX memory
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    On 07.07.22 16:26, Dave Hansen wrote:
    > On 6/26/22 23:16, Kai Huang wrote:
    >> On Fri, 2022-06-24 at 12:40 -0700, Dave Hansen wrote:
    >>>> +/*
    >>>> + * Walks over all memblock memory regions that are intended to be
    >>>> + * converted to TDX memory. Essentially, it is all memblock memory
    >>>> + * regions excluding the low memory below 1MB.
    >>>> + *
    >>>> + * This is because on some TDX platforms the low memory below 1MB is
    >>>> + * not included in CMRs. Excluding the low 1MB can still guarantee
    >>>> + * that the pages managed by the page allocator are always TDX memory,
    >>>> + * as the low 1MB is reserved during kernel boot and won't end up to
    >>>> + * the ZONE_DMA (see reserve_real_mode()).
    >>>> + */
    >>>> +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid) \
    >>>> + for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid) \
    >>>> + if (!pfn_range_skip_lowmem(p_start, p_end))
    >>>
    >>> Let's summarize where we are at this point:
    >>>
    >>> 1. All RAM is described in memblocks
    >>> 2. Some memblocks are reserved and some are free
    >>> 3. The lower 1MB is marked reserved
    >>> 4. for_each_mem_pfn_range() walks all reserved and free memblocks, so we
    >>> have to exclude the lower 1MB as a special case.
    >>>
    >>> That seems superficially rather ridiculous. Shouldn't we just pick a
    >>> memblock iterator that skips the 1MB? Surely there is such a thing.
    >>
    >> Perhaps you are suggesting we should always loop the _free_ ranges so we don't
    >> need to care about the first 1MB which is reserved?
    >>
    >> The problem is some reserved memory regions are actually later freed to the page
    >> allocator, for example, initrd. So to cover all those 'late-freed-reserved-
    >> regions', I used for_each_mem_pfn_range(), instead of for_each_free_mem_range().
    >
    > Why not just entirely remove the lower 1MB from the memblock structure
    > on TDX systems? Do something equivalent to adding this on the kernel
    > command line:
    >
    > memmap=1M$0x0
    >
    >> Btw, I do have a checkpatch warning around this code:
    >>
    >> ERROR: Macros with complex values should be enclosed in parentheses
    >> #109: FILE: arch/x86/virt/vmx/tdx/tdx.c:377:
    >> +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid) \
    >> + for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid) \
    >> + if (!pfn_range_skip_lowmem(p_start, p_end))
    >>
    >> But it looks like a false positive to me.
    >
    > I think it doesn't like the if().

    I think it is right.

    Consider:

    if (a)
    memblock_for_each_tdx_mem_pfn_range(...)
    func();
    else
    other_func();


    Juergen
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