Re: [PATCH v11 11/20] x86/virt/tdx: Fill out TDMRs to cover all TDX memory regions

[Huang, Kai]

On Wed, 2023-06-14 at 15:31 +0300, Nikolay Borisov wrote:
> 
> On 4.06.23 г. 17:27 ч., Kai Huang wrote:
> <snip>
> 
> > ---
> >   arch/x86/virt/vmx/tdx/tdx.c | 94 ++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 93 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> > index 7a20c72361e7..fa9fa8bc581a 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.c
> > +++ b/arch/x86/virt/vmx/tdx/tdx.c
> > @@ -385,6 +385,93 @@ static void free_tdmr_list(struct tdmr_info_list *tdmr_list)
> >   			tdmr_list->max_tdmrs * tdmr_list->tdmr_sz);
> >   }
> >   
> > +/* Get the TDMR from the list at the given index. */
> > +static struct tdmr_info *tdmr_entry(struct tdmr_info_list *tdmr_list,
> > +				    int idx)
> > +{
> > +	int tdmr_info_offset = tdmr_list->tdmr_sz * idx;
> > +
> > +	return (void *)tdmr_list->tdmrs + tdmr_info_offset;
> 
> nit: I would just like to point that sizeof(void *) being treated as 1 
> is a gcc-specific compiler extension: 
> https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Pointer-Arith.html#Pointer-Arith
> 
> I don't know if clang treats it the same way, just for the sake of 
> simplicity you might wanna change this (void *) to (char *).

Then we will need additional cast from 'char *' to 'struct tdmr_info *' I
suppose?  Not sure whether it is worth the additional cast.

And I found such 'void *' arithmetic operation is already used in other kernel
code too, e.g., below code in networking code:

	./net/rds/tcp_send.c:105:     (void *)&rm->m_inc.i_hdr + hdr_off,

and I believe there are other examples too (that I didn't spend a lot of time to
grep).

And it seems Linus also thinks "using arithmetic on 'void *' is generally
superior":

https://lore.kernel.org/lkml/CAHk-=whFKYMrF6euVvziW+drw7-yi1pYdf=uccnzJ8k09DoTXA@xxxxxxxxxxxxxx/t/#m983827708903c8c5bddf193343d392c9ed5af1a0

So I wouldn't worry about the Clang thing.