Re: [PATCH v7 16/20] x86/virt/tdx: Configure TDX module with TDMRs and global KeyID

[Huang, Kai]

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.