On Thu, 2022-11-24 at 17:18 -0800, Dave Hansen wrote: > On 11/24/22 16:59, Huang, Kai wrote: > > On Wed, 2022-11-23 at 15:56 -0800, Dave Hansen wrote: > > > On 11/20/22 16:26, Kai Huang wrote: > > > > + 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? > > > > Correct. > > > > > 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? > > > > The alignment is assumed to be guaranteed based on the Documentation you quoted > > below. > > I'm feeling kinda dense today, being Thanksgiving and all. Could you > please walk me through, step-by-step how you get kzalloc() to give you > an allocation where the start address is 512-byte aligned? Sorry I am not good at math. I forgot although 512 is power of two, n x 512 may not be power of two. The code works because in practice tdmr_num is never larger than 64 so tdmr_num * sizeof(64) cannot be larger than 512. So if want to consider array size being larger than 4K, we should use alloc_pages_exact() to allocate? > > ... > > Perhaps I should just allocate one page so it must be 512-byte aligned? > > > > /* > > * TDMR_INFO entries are configured to the TDX module via an array > > * of physical address of each TDMR_INFO. The TDX module requires > > * the array itself to be 512-byte aligned. Just allocate a page > > * to use it as the array so the alignment can be guaranteed. The > > * page will be freed after TDH.SYS.CONFIG anyway. > > */ > > Kai, I just plain can't believe at this point that comments like this > are still being written. I _thought_ I was very clear before that if > you have a constant, say: > > #define FOO_ALIGN 512 > > and you want to align foo, you can just do: > > foo = ALIGN(foo, FOO_ALIGN); > > You don't need to mention the 512-byte alignment again. The #define is > good enough. > > My fault. I thought by changing to allocate a page we don't need to do 'foo = ALIGN(foo, FOO_ALIGN)' so I thought the comment could be useful. Thanks for responding at Thanksgiving day.
