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    SubjectRe: [PATCH v7 16/20] x86/virt/tdx: Configure TDX module with TDMRs and global KeyID
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    On 11/20/22 16:26, Kai Huang wrote:
    > After the TDX-usable memory regions are constructed in an array of TDMRs
    > and the global KeyID is reserved, configure them to the TDX module using
    > TDH.SYS.CONFIG SEAMCALL. TDH.SYS.CONFIG can only be called once and can
    > be done on any logical cpu.
    >
    > Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
    > Signed-off-by: Kai Huang <kai.huang@intel.com>
    > ---
    > arch/x86/virt/vmx/tdx/tdx.c | 37 +++++++++++++++++++++++++++++++++++++
    > arch/x86/virt/vmx/tdx/tdx.h | 2 ++
    > 2 files changed, 39 insertions(+)
    >
    > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
    > index e2cbeeb7f0dc..3a032930e58a 100644
    > --- a/arch/x86/virt/vmx/tdx/tdx.c
    > +++ b/arch/x86/virt/vmx/tdx/tdx.c
    > @@ -979,6 +979,37 @@ static int construct_tdmrs(struct tdmr_info *tdmr_array, int *tdmr_num)
    > return ret;
    > }
    >
    > +static int config_tdx_module(struct tdmr_info *tdmr_array, int tdmr_num,
    > + u64 global_keyid)
    > +{
    > + u64 *tdmr_pa_array;
    > + int i, array_sz;
    > + u64 ret;
    > +
    > + /*
    > + * TDMR_INFO entries are configured to the TDX module via an
    > + * array of the physical address of each TDMR_INFO. TDX module
    > + * requires the array itself to be 512-byte aligned. Round up
    > + * the array size to 512-byte aligned so the buffer allocated
    > + * by kzalloc() will meet the alignment requirement.
    > + */

    Aagh. Return of (a different) 512-byte aligned structure.

    > + array_sz = ALIGN(tdmr_num * sizeof(u64), TDMR_INFO_PA_ARRAY_ALIGNMENT);
    > + tdmr_pa_array = kzalloc(array_sz, GFP_KERNEL);

    Just to be clear, all that chatter about alignment is because the
    *START* of the array has to be aligned. Right? I see alignment for
    'array_sz', but that's not the start of the array.

    tdmr_pa_array is the start of the array. Where is *THAT* aligned?

    How does rounding up the size make kzalloc() magically know how to align
    the *START* of the allocation?

    Because I'm actually questioning my own sanity at this point, I went and
    double-checked the docs (Documentation/core-api/memory-allocation.rst):

    > The address of a chunk allocated with `kmalloc` is aligned to at least
    > ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
    > alignment is also guaranteed to be at least the respective size.

    Hint #1: ARCH_KMALLOC_MINALIGN is way less than 512.
    Hint #2: tdmr_num is not guaranteed to be a power of two.
    Hint #3: Comments don't actually affect the allocation

    <snip>

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