Re: [PATCH v8 09/16] x86/virt/tdx: Fill out TDMRs to cover all TDX memory regions

[Huang, Kai]

On Fri, 2023-01-06 at 11:36 -0800, Dave Hansen wrote:
> On 12/8/22 22:52, Kai Huang wrote:
> > Start to transit out the "multi-steps" to construct a list of "TD Memory
> > Regions" (TDMRs) to cover all TDX-usable memory regions.
> > 
> > The kernel configures TDX-usable memory regions by passing a list of
> > TDMRs "TD Memory Regions" (TDMRs) to the TDX module.  Each TDMR contains
> > the information of the base/size of a memory region, the base/size of the
> > associated Physical Address Metadata Table (PAMT) and a list of reserved
> > areas in the region.
> > 
> > Do the first step to fill out a number of TDMRs to cover all TDX memory
> > regions.  To keep it simple, always try to use one TDMR for each memory
> > region.  As the first step only set up the base/size for each TDMR.
> > 
> > Each TDMR must be 1G aligned and the size must be in 1G granularity.
> > This implies that one TDMR could cover multiple memory regions.  If a
> > memory region spans the 1GB boundary and the former part is already
> > covered by the previous TDMR, just use a new TDMR for the remaining
> > part.
> > 
> > TDX only supports a limited number of TDMRs.  Disable TDX if all TDMRs
> > are consumed but there is more memory region to cover.
> 
> This could probably use some discussion of why it is not being
> future-proofed.  Maybe:
> 
> 	There are fancier things that could be done like trying to merge
> 	adjacent TDMRs.  This would allow more pathological memory
> 	layouts to be supported.  But, current systems are not even
> 	close to exhausting the existing TDMR resources in practice.
> 	For now, keep it simple.

Looks great.  Thanks.

> 
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> > index d36ac72ef299..5b1de0200c6b 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.c
> > +++ b/arch/x86/virt/vmx/tdx/tdx.c
> > @@ -407,6 +407,90 @@ 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)
> > +{
> > +	return (struct tdmr_info *)((unsigned long)tdmr_list->first_tdmr +
> > +			tdmr_list->tdmr_sz * idx);
> > +}
> 
> I think that's more complicated and has more casting than necessary.
> This looks nicer:
> 
> 	int tdmr_info_offset = tdmr_list->tdmr_sz * idx;
> 
> 	return (void *)tdmr_list->first_tdmr + tdmr_info_offset;
> 
> Also, it might even be worth keeping ->first_tdmr as a void*.  It isn't
> a real C array and keeping it as void* would keep anyone from doing:
> 
> 	tdmr_foo = tdmr_list->first_tdmr[foo];

Yes good point.  Will do.

[snip]


> 
> Otherwise this actually looks fine.

Thanks.